Download Elevated 87Sr/86Sr Ratios From Mafic Intrusions in the Atlanta Lobe

Elevated 87Sr/86Sr Ratios
From Mafic Intrusions in the
Atlanta Lobe of the Idaho Batholith
Charles W. Russell
(Deceased)
Janet Gabites
Geological Sciences 309
University of British Columbia
Vancouver, British Columbia
Publication Note:
The full study following this title page appears as it was transmitted by Charlie
Russell to the Idaho Geological Survey on September 3, 2002, ten days before he
died from complications related to multiple sclerosis. The content is provided here
unedited by the Idaho Geological Survey. Additional information is available in
his Ph.D. dissertation:
Russell, Charles W., 1988, Crystallization history of the Banks complex:
Implications for middle crustal evolution in Cordilleran batholithic terranes:
University of Washington Ph.D. dissertation, 226 p.
Idaho Geological Survey
University of Idaho
Moscow, Idaho 83844-3014
Technical Report 05-1
2005
This document has been formatted for onscreen viewing using either Adobe Acrobat,
or the Adobe Acrobat reader.
If printed, it will be approximately 60 pages
in length.
Printing may not be necessary.
1.
All figures and tables have been collected at the
end of the text, but are accessible within the text
from the blue boxes. Once you are done examining a figure, if you "right-click" on a figure,
you will bring up a menu box with "Go to previous view" as one of the options. Use of this option will allow you to quickly switch between figures and text.
2.
Figures have been reduced in size and strongly
compressed to limit download times - larger
versions of these figures are available upon request. Note that Adobe Acrobat will allow you
to magnify a figure up to 200%. For line drawings, all relevant detail should be recoverable
using this method (some photographic detail
may not be recoverable).
Elevated 87Sr/86Sr ratios from mafic intrusions in the Atlanta Lobe
of the Idaho batholith
Charles W. Russell
Janet Gabites
Geological Sciences 309, University of British Columbia, Vancouver, BC
[ABSTRACT]
Mafic intrusions (tonalites, quartz diorites, diorites, and gabbros), which range from 1 to 2 km
in length, and vary in width from 50 to 300m, are located within the 95 Ma granodiorite of the
border zone of the Idaho batholith near Banks, Idaho. These intrusions were emplaced at midcrustal depths (10 - 18 km), prior to final crystallization of the host granodiorite. The mafic intrusions were themselves intruded by consanguineous, synplutonic, fine-grained, hornblende
diorite dikes. Although the [87Sr/86Sr]i values for the granodiorites, which range from 0.7054 0.7067, are similar to other border zone rocks of the Idaho batholith, the mafic rocks have distinctly higher (87Sr/86Sr)i values of 0.7071 - 0.7084, high Sr concentrations (1400 - 1700 ppm),
and cannot be linked by simple fractional crystallization to the granodiorites. However, it is
possible to model the intermediate values of the quartz diorites and tonalites of the mafic intrusions through mixing and hybridization with the granodiorite.
Although the elevated [87Sr/86Sr]i values of the mafic rocks suggest lower crustal contamination,
there are only weak negative Eu anomalies (Eu/Eu* = 0.83 to 0.91) in the fine-grained diorite,
and these are overshadowed by moderate positive Eu anomalies (Eu/Eu* = 1.16 to 1.79) in the
coarse-grained mafic rocks. Together with the high Sr values, these data preclude equilibrium
with plagioclase bearing rocks at greater depth. Consequently, if the Sr isotope signature for
the mafic magmas is derived by the melting of sialic lower crustal rocks, that process would
seem to require disequilibrium total fusion along magma conduits, with initial mafic magma
values in excess of 2000 ppm Sr. It is inferred that a more likely the source for the crustal Srcomponent may be LILE-enriched fluids driven off of the subducting slab, which produced
melting in the overlying mantle wedge. Similarly, the depletion of the normative ferromagnesian mineral components in the mafic magmas seems more likely to be the result of fractional
crystallization within the mantle, rather than a crustal process, because of the lack of bulk
chemical and trace element signatures for crustal assimilation. These observations underscore
the diversity of sources, and the complexity of the processes, which are involved in the genesis
of Cordilleran batholiths.
...mafic intrusions in the Atlanta Lobe...
1.)
Introduction
a)
Background
Page 2
The large Cordilleran batholiths which fringe the western margins of North American and South
American continents are some of the most obvious examples of recent genesis and reworking of
continental crust, and their origins are of fundamental geologic importance. The volumetric dominance of monzogranites, granodiorites, and tonalites in these batholiths was recognized very early.
However, it was not until the latter portion of the twentieth century that contemporaneous mafic
dikes and plutons were recognized as important sources of mantle-derived heat, and material for the
felsic magmas. Examples of close spatial and temporal associations for felsic and mafic magmas
have been described in the Peruvian Coastal batholith (Pitcher and Bussell, 1985), the Sierra Nevada batholith (Reid and others, 1983; Furman and Spera, 1985; Frost and Mahood, 1987; Coleman
and others, 1995, Ratajeski and others, 2001), and the Idaho batholith (Hyndman and Foster, 1988;
Foster and Hyndman, 1991). Moreover, the close association of mafic magmas with granites is not
restricted to the Mesozoic Cordilleran batholiths. Similar spatial and temporal associations are seen
in Paleozoic magmatic provinces in Antarctica (Di Vincenzo and Rocchi, 1999), Europe (Altherr
and others, 1999), and eastern North America (Wiebe and others, 1998).
Coincident with these observations on the spatial and temporal association of mafic magmas with
the granitic rocks, was the realization that rather than being simple crustal melts, most of the granitic rocks formed from - or evolved in close association with - these mafic magmas through complex processes of assimilation, mixing, hybridization, and fractional crystallization. These observations support a view of Cordilleran batholiths which considers them to be petrologically complex
and diverse entities, despite the large bodies of apparently homogeneous granitic rocks that make
up their bulk. In this context the mafic magmas are of particular interest because they contain geochemical information - albeit, incomplete - about magmagenesis and early magmatic evolution
deep below the crustal manifestation of the batholith. Such information is difficult, if not impossible, to recover from the associated granitic rocks, where the high-level crustal processes reduce or
erase geochemical relief. Consequently, the mafic magmas provided the best opportunity to understand some of the fundamental processes that shape the evolution of the large Cordilleran batholiths, and their more voluminous granitic rocks.
In this study we present data on mafic intrusions, which appear to have been intruded late in the
crystallization history of their host granodiorites, on the western margin of the Idaho batholith. The
results reported here are consonant with, and reinforce, the observations offered above regarding
the association of mafic and felsic magmas. However, as the rocks which are described in this paper have geochemical characteristics which are unusual, they underscore the larger themes in Cordilleran batholith development, which are diversity in sources, and complex evolutionary histories.
b)
Regional Geology
The Idaho batholith is Cretaceous in age and is separated by Precambrian rocks of the Salmon
River arch into two lobes, the Bitterroot in the north and the Atlanta in the south (Figure 1). The
western boundary of the batholith marks the most westward extent of the Precambrian craton.
Farther west lie the allochthonous terranes of eastern Oregon and western Idaho, which were accreted to North America during the Cretaceous. These terranes have been intruded by Permian to
Interim Draft for Preliminary Review
[email protected]
Charles W. Russell
...mafic intrusions in the Atlanta Lobe...
Page 3
Cretaceous (Fleck and Criss, 1985) trondhjemites, tonalites, and gabbros to produce the satellite
plutons. Dates on these plutons indicate that the terranes were accreted from 130 to 75 Ma, and
that the border zone plutons of the batholith (see below) "stitched" the terranes to the continent
during the period of 95 to 80 Ma (Fleck and Criss, 1985).
East of the satellite plutons is the border zone of the batholith, which is a 10 to 20 km wide zone of
"I-type" tonalites and granodiorites (Hyndman, 1983). The contrast between the border zone rocks
and the satellite plutons is most obvious in the Sr isotope values, which show that the transition
from the older satellite plutons to border zone rocks is accompanied by an increase in 87Sr/86Sr from
0.704 to 0.706 over a short distance (Armstrong and others, 1977; Fleck and Criss, 1985). This
"0.704-0.706" line is equivalent to the "quartz diorite line" in western Idaho (Moore, 1959) and the
increase in 87Sr/86Sr ratio is attributed to assimilation of Precambrian crustal material (Armstrong
and others, 1977; Fleck and Criss, 1985). Snee and others (1995) review evidence for depths of at
least 20 km for border zone tonalites in the Slate Creek area (150 km north and west of the study
area for this paper), and such depths are consonant with the general inference that Idaho batholith
plutons show deeper levels of exposure than do plutons from the better studied Sierran Nevada
batholith. Additional work on the border zone rocks of the Atlanta lobe includes regional summaries and reconnaissance studies by Anderson (1952), Larsen and Schmidt (1958), Schmidt (1964),
Taubeneck (1971), and Lewis and others (1987), which described the general nature of the batholithic rocks, and Goodspeed (1973), which speculated on the possible origin of orbicules in some of
the rocks near Banks.
Further inboard of the border zone units - and beyond the focus of this study - are the voluminous
monzogranites, and granodiorites that constitute the main phase units of the Idaho batholith. These
rocks are peraluminous granodiorites and monzogranites, most of which lack hornblende and contain relatively little biotite (<5%). Contacts between units are diffuse and poorly defined, and deformation during and beyond the latest stages of crystallization produced granitic rocks with pronounced protoclastic and cataclastic textures, particularly within the Bitterroot Lobe (Reid and others; 1979; Hyndman, 1984; Reid, 1987). Other notable features of the main phase units are local
abundance of migmatites (Myers, 1982; Bittner, 1987), and high [87Sr/86Sr]i (> 0.708) relative to
other Cordilleran plutons (Shuster and Bickford, 1985; Fleck and Criss, 1985). Because of their
relatively high Na2O, the main phase units have been described as being "transitional I/S-type"
granites (Hyndman, 1983). In comparison with the low (87Sr/86Sr)i, metaluminous, and hornblende
bearing rocks of the border zone, the granites and granodiorites of the main phase units have
higher (87Sr/86Sr)i values, and many are peraluminous and muscovite bearing (Hyndman, 1983,
1984; Kilsgaard and Lewis, 1985; Lewis and others, 1987; Reid, 1987). General similarities in
compositions are noted for the main phase units of the Atlanta Lobe and the Bitterroot Lobe (Lewis
and others, 1987), but the Atlanta Lobe lacks the small-scale structures which suggest synkinematic
intrusion of the Bitterroot Lobe (Reid, 1987).
The conclusion of most workers has been that the main phase units resulted from large-scale, crustal anatexis in which heat, and a variable amounts of material, have been supplied by mantle magmas (Hyndman, 1981; Bickford and others, 1981; Shuster and Bickford, 1985; Fleck and Criss,
1985; Hyndman and Foster, 1988). Various researchers have speculated that the main phase units
of the Idaho batholith were emplaced at mesozonal to catazonal depths (Buddington, 1959; Hamilton, 1981; Hyndman, 1984), although, Snee and Lund (1988) argue that at least some of these
granites were emplaced at depths as shallow as 9 km. Ages for the main phase units vary from late
Interim Draft for Preliminary Review
[email protected]
Charles W. Russell
...mafic intrusions in the Atlanta Lobe...
Page 4
Cretaceous to early Tertiary. A period of diminished magmatic activity during the early Tertiary
separated the main phase units of the batholith from the middle Eocene plutonic rocks of the Challis event (Bennett, 1980).
c)
Research Outline
A study area was selected near Banks, Idaho, on the western margin of the Atlanta Lobe, where the
canyons at the confluence of the North Fork and Middle Fork of the Payette River provide exposures of pods of gabbroic and dioritic magma injected into the more voluminous granodiorite of the
border zone of the Idaho batholith (Figure 2). The main objective was to produce a detailed geochemical traverse (Line ABCD on Figure 2) across one of the larger mafic intrusions (transected by
segment BC on Figure 2, and termed here, the "Banks intrusion") approximately two miles north of
the town of Banks, and understand the relationship between the intrusion and the host granodiorite.
Emphasis during mapping was on observation and recording of contact relations between the various rocks units, and discrimination of key textural features which might bear upon petrologic interpretations. To this end, information on modes, whole rock geochemistry, trace elements, rare earth
elements, and Sr isotope variations across a suite of rocks was collected (Figure 3; Table 1). The
details of analytical procedures that were used are contained in the appendix.
The Banks area was selected because the major structures and dominant textures in these rocks
were believed to result from igneous process, and not from subsequent metamorphic or hydrothermal effects. However, there are perceptible post-magmatic features in these rocks. Deuteric alteration ranges from weak in the granodiorite (less than 10% alteration of the ferromagnesian minerals to chlorite), to moderate in the mafic intrusions (greater than 20% alteration of ferromagnesian minerals to chlorite + epidote + sphene ± actinolite ± iron sulphide in the gabbros and diorites).
In addition, there is pronounced potassic alteration adjacent to some fracture zones, which is recognized as a complete replacement of the host rock by potassium feldspar, sphene, chlorite, and epidote in a 10-cm wide selvage. Although much of the allanite in the granodiorite appears magmatic
(see below), in the mafic rocks some of the allanite is in interstitial lobate masses which grade into
pistacite, and these relations indicate that a portion of the allanite in these rocks may be deuteric
(Exley, 1980). Only in the mafic rocks is there any appreciable alteration (<5%) of feldspars to
sericite.
The dynamothermal metamorphism and tectonic dismemberment of rock units which is a hallmark
of allochthonous terranes to the north in Salmon river area, is not evident within the confines of this
study area. Although several northwest trending fractures can be identified in the study area, these
fractures show limited development of gouge (typically less than 2 cm), and no appreciable offsets.
Where planar fabrics can be defined, they wrap around the margins of the mafic intrusions, and appear to reflect late igneous movements, rather than a regional penetrative deformation. Deformation is suggested in some of the foliated tonalites and granodiorites where clusters of biotite grains
wrap around the larger feldspar grains: however, few grains show unresolved strain. Within the
granodiorites, some quartz grains show moderate undulatory extinction or polygonization, and the
development of some minor mortar zones. However, the lack of strained, bent, or broken minerals
(other than minor strain within the quartz, and some mechanical twinning in the plagioclases), and
the widespread survival of igneous textures and structures suggest that the stresses which produced
the planar fabric did not persist for a significant period of time following the crystallization of the
magma.
Interim Draft for Preliminary Review
[email protected]
Charles W. Russell
...mafic intrusions in the Atlanta Lobe...
Page 5
Efforts have been made to reference suitable locations where key structures, lithologies, and textures can be examined and sampled by other researchers. Such locations are reported using the
shortened 10-meter coordinates for the 0548 block of UTM zone 11 (i.e. - the coordinates for the
southeast corner of the study area at Zone 11, 057000488100 are reported as 70008100, and the coordinates for the northeast corner of the study area at Zone 11, 057500488600 are reported as
75008600), which were derived by map inspection. For those researchers who are more comfortable with a degree-based coordinate system, we note that the southwest corner of the study area is
Longitude 116°7.589' W, Latitude 44°4.858' N, and the northeast corner is Longitude 116°3.785'
W, Latitude 44°7.501' N.
Interim Draft for Preliminary Review
[email protected]
Charles W. Russell
...mafic intrusions in the Atlanta Lobe...
2.)
Field Observations and Petrography
a)
Structural Relations
Page 6
The bulk of the project area is underlain by granodiorite, which has been intruded by northeasttrending, pod-shaped mafic bodies that range from 1 to 2 km in length, and vary in width from 50
to 300m. The mafic intrusions are composed of heterogeneous mixtures of tonalite, quartz diorite,
diorite, and gabbro, and display a wide variety of textures. Related to the more distinct mafic intrusions are numerous small pods of hornblende-bearing quartz diorite and tonalite that vary in length
from 5 to 300 meters, and are too poorly exposed to allow detailed delineation. On the smallest
scale (15 to 50 cm) are the "white zones" which are lobate pods of plagioclase (85%) and hornblende (15%) within the granodiorites and tonalites (Figure 4). Some these rocks - especially the
tonalites and granodiorites on the margins of the mafic intrusions - show a distinct foliation, which
is defined by oriented biotite grains, oblong mafic inclusions, schlieren, and late quartz segregations. A plot of the poles to the foliation shows the northwest trend, and the near vertical character
which is noted in the field (Figure 5), but also the shallower dipping fabrics from the northeastern
edge of the largest of the mafic intrusion, where the foliation wraps around the body.
Contacts of the mafic intrusions with the granodiorite are exposed in few places, and cannot be
traced for more than several meters in any locality. Moreover, a variety of contact relationships can
be observed in those locations where they are exposed. A sharp boundary between the hornblendebiotite tonalite of one of the smaller bodies of mafic intrusive (immediately southwest of one of the
larger masses of mafic intrusion shown on Figure 2) and the biotite granodiorite country rock can
be seen at 71398199 (Figure 6). Gradational contacts of the hornblende quartz diorite of a mafic
intrusion with the biotite granodiorite country rock are exposed at 72498368. At this location the
contact is manifested as a series of decimeter-scale intercalations granodiorite and tonalite/quartz
diorite, in which the two phases retain a distinct boundary, but megacrystic hornblende from the
mafic complex diorites can be found in the otherwise typical granodiorite. There are also distinct
"white zones" in the granodiorite, and wispy, 2-cm interdigitations of granodiorite with the tonalite
and quartz diorite (Figure 7). In a given intercalation the contact between the quartz diorite and
granodiorite occurs over less than 10 mm, but is diffuse and no sharp boundary separates the two.
Such a diffuse contact can be demonstrated is 72518374, where granodiorite with swirling schlieren
grades into tonalite with an increase in hornblende over a 100-meter transition zone. Throughout
the study area, where exposure is less good, only the sharply delineated contacts can be identified,
and the subtle variations of the other types of contacts are lost. However, judging from the manifestations of lithologic diversity within the intrusions and adjacent to them, diffuse contacts may
well be the most common throughout the Banks area.
Pegmatites (quartz-orthoclase-plagioclase-biotite) are common in the project area, and some contain rounded anhedral spessartine garnet (74938369). Both the tabular and bulbous pegmatite categories of Brisbin (1986) occur, and many have contacts that are gradational rather than sharp. In
some locations (such as 74008326 and 72578250) the pegmatites are tabular sheets on the scale of
centimeters to meters, but on a larger scale they pinch and swell, so that over the course of several
meters their general form persists as lenses of pegmatitic material and individual potassium feldspar megacrysts dispersed throughout the intervening granodiorite. It is inferred that the volatile
phases from which these pegmatites crystallized exsolved prior to complete crystallization of the
Interim Draft for Preliminary Review
[email protected]
Charles W. Russell
...mafic intrusions in the Atlanta Lobe...
Page 7
granodiorite, and that they underwent synplutonic dismemberment prior to, and during their crystallization.
A plot of poles from those pegmatites which lie within well-defined fracture planes indicates a
northwest trend with near vertical orientation is most common (Figure 7). This orientation is similar to what would be expected for the extension fractures that would be produced by the emplacement of mafic magmas along the southwest to northeast axis. Unfortunately, although there is a
obvious temporal relationship between the dissected pegmatites and the granodiorite, the pegmatites that fill the northwest-trending fractures cannot be unequivocally tied to the granodiorites.
Consequently, both temporal and genetic linkages between the fracture filling pegmatites and the
granodiorites and the mafic intrusions remain speculative.
b)
Granodiorites
Although the granodiorite is described here as one lithology, there may well be several masses of
magma which are represented by this unit. Textures vary considerably across the study area.
Southwest of the project area, the granodiorite is distinctly granoblastic. Immediately to the northwest, there are migmatitic varieties of granodiorite, with stromatic migmatites apparent in some
outcrops. Although discrimination of textural variants is possible, the subtle variations in grain
morphology occur gradually, and frustrate any attempts to draws discrete contacts between units.
Perhaps more importantly, there a appreciable geochemical differences (see following section) in
some granodiorites which are not easily discriminated on the basis of textural variation, an observation which underscores the limitations of any taxonomy based upon textural characteristics. Distinct contacts between these texturally distinctive granodiorites, and the more common allotriomorphic granular granodiorites in the area of the Banks complex, are not evident.
Nonetheless, within the area of the mafic complexes, there are certain attributes that characterized
the granodiorite. The rock is dominantly medium-grained, equant, subhedral plagioclase (weak
normal zoning from An30 to An35) and less abundant quartz. The potassium feldspar is either in
interstices of the plagioclase-rich matrix as anhedral masses of orthoclase, or as megacrysts (perthitic orthoclase and microcline) with forms that vary from anhedral to subhedral. Development of
myrmekite is common between the potassium and plagioclase feldspars. The rock has a color index of from 3 - 7, which is due to shreddy clots and masses of brown biotite. Although in places
the rock has a foliation defined by weakly oriented biotite grains, or schlieren, in most outcrops it is
a massive, homogeneous material. Hornblende, magnetite, sphene, and epidote-allanite are present
as accessory minerals in the granodiorite, and trace minerals include sphene and apatite. Although
the accessory and trace minerals are concentrated in the clots of biotite, some particular associations of the sphene and epidote are important to note. Some of the sphene is in distinct nodules (3
to 8 mm semispherical patches of plagioclase that are cored by a single sphene grain, which is typically displayed embayed) are a distinctive and common feature of the granodiorite, especially in the
areas between some of the mafic intrusions. Although much of the epidote is a pistacite with a
light green pleochroism, the cores of some grains are subhedral to euhedral brown allanite grains,
and many of the pistacite-rich grains mantle subhedral to euhedral allanites.
Although uncommon, some apparently metasedimentary xenoliths can be found in the granodiorite.
Some garnet-pyroxene xenoliths (at 73008423 and 71908417) contain 30% garnet, 50% pyroxene,
and 20 % quartz. Xenoliths of garnet-biotite-sillimanite schist occur as lenses in the granodiorite
over a 50 m2 area at 74608580. The grain sizes in these xenoliths range from fine to coarse-grained,
Interim Draft for Preliminary Review
Charles W. Russell
[email protected]
...mafic intrusions in the Atlanta Lobe...
Page 8
and the rocks weather a distinctive rusty brown. These xenoliths are composed of 30% coarsegrained, subhedral garnet, 30% medium-grained, reddish-brown biotite, and 40% fine- to mediumgrained prismatic sillimanite. Quartz and feldspars are completely lacking, and together with xenocrystic garnets in the adjacent granodiorite, this suggests partial fusion, and some dismemberment
of the xenoliths. Beside these smaller xenoliths, there is one large block of quartzite within the
granodiorite, which can be mapped on the basis of float that is distributed over approximately a 100
m by 100 m area, centered upon 73338519
With the exception of agmatitic granodiorite at 73338519, mafic inclusions and enclaves are uncommon in the granodiorite, and are seldom present in "swarms" such as identified elsewhere
(Eichelberger, 1980; Frost and Mahood, 1987). However, "white zones" (pods and patches of diorite composed of 80-90% plagioclase and 10-20% hornblende), which range in size from 10 cm to
several meters, are distributed throughout the granodiorite in locations near the mafic intrusions.
Within the "white zones" the hornblende grains have numerous 0.3 to 0.7 mm, poikilitic inclusions
of quartz and subhedral plagioclase, which give the grains a sieve-like appearance. Where the
"white zones" are small, they are elongate, nebular bodies with apophyses that extend out into the
granodiorite. Larger bodies have gently curving contacts with the granodiorite. Contacts with the
granodiorite are diffuse, but the transition between lithologies is accomplished within several millimeters.
Besides the contemporaneous pegmatites and mafic intrusions, which intrude the granodiorite,
there are some other dikes rocks. A single, two-meter, fine-grained mustard-brown dike trending
northwest and dipping northeast is present at 72518369, and float from a similar appearing rock
was found 2.5 km east at 74898477. At the first locality the dike cuts across foliation, and has
sharp contacts with the host tonalites, and these rocks are suspected to be later (possibly Challisage) intrusions. There was also one unusual quartz monzodiorite (CWR 1385), which was collected from a near-horizontal tabular mass (a "dike"), which had transitional rather than sharp contacts with the granodiorite. Texturally, this rock is dominated by medium- to coarse-grained potassium feldspar phenocrysts, around which masses of biotite swirl. This is a strongly alkalic rock,
which plots well off of the cotectic curves in normative q-an-ab-or space, and does not appear to
represent a magmatic liquid that would be related to any of the other magmas in this suite. Although its origin is unclear, it may be a late synplutonic dike, which shows metasomatic impacts
from mingling with fluids from some of the earliest exsolved pegmatites.
c)
Tonalites
Although in some places the granodiorite country rock is in contact with the coarse-grained quartz
diorites of the mafic complexes, along many of the contacts hornblende-biotite tonalite - or less
common biotite tonalite - lies between the two as a transitional lithology. Many of these tonalites
have a weak foliation that is most obvious as elongate zones of interconnected interstitial quartz
and parallel clusters of biotite flakes. There is also a very uncommon, weak hornblende lineation.
Mafic inclusions and "white zones" are more typical of these rocks than of the granodiorites. Plagioclase grains in the tonalites have compositions in the range of An36 to An42. Larger grains have
diffuse patchy zoning, and may have inclusions of trapezoidal to rectangular alkali feldspar, as well
as subhedral to nearly euhedral epidote, besides other accessory minerals. Coarse-grained, subhedral to euhedral hornblende phenocrysts are one of the most striking features of these rocks. However, the bulk of the mafic minerals are distributed in 3 to 8 mm clusters of biotite, hornblende,
Interim Draft for Preliminary Review
[email protected]
Charles W. Russell
...mafic intrusions in the Atlanta Lobe...
Page 9
oxide, and sphene. Inclusions of ilmenite, biotite, quartz, zircon, and apatite are common in the
hornblende, and some biotite grains have inclusions of epidote, apatite, oxides, and quartz.
There is a perceptible difference in the amount and morphology of the various accessory and trace
minerals in these rocks relative to the granodiorites. The opaque phase is more abundant in the tonalite, and apatite and zircon are prominent. However, both sphene and epidote are less common
than in the granodiorites, with sphene nodules absent, and allanite-pistacite mantling and zoning
less striking. The sphene is present as mantles on oxides, and less commonly as mantles on biotite,
or intergrowths with hornblende. Many sphene grains are deeply embayed by plagioclase, and
some are included within biotite (and less commonly, within hornblende). Although some hornblende grains contain subhedral ilmenite inclusions, the major opaque phase in the rock is magnetite, and many of these grains are mantled with sphene.
Although these rocks contain some epidote as an alteration product as selvages along some faults,
there is a population of epidote grains which is believed to be magmatic in origin. Where distinct
epidote grains are not contained in, or intimately intergrown with, other minerals, they are present
in anhedral to subhedral forms that range from 0.3 to 0.7 mm in length. These grains do not have
morphological features that serve to establish either a magmatic or a deuteric origin. However,
some grains of epidote are intergrown as optically continuous, interdigitating masses with hornblende or plagioclase. Moreover, although much of the epidote is a pistacite with a light green
pleochroism, the cores of some grains are subhedral to euhedral brown allanite grains, and many of
the pistacite grains mantle subhedral to euhedral allanites. Indeed, in some rocks, euhedral pistacite
grains, which mantle allanite, are themselves mantled with biotite. Moreover, euhedral epidote inclusions within plagioclase can be found. These latter two textures strongly suggest that the epidote is a primary magmatic mineral (Zen and Hammarstrom, 1984; Evans and Vance, 1987), albeit
with a potentially high REE content.
d)
Coarse-grained quartz diorite of the mafic intrusions
The interiors of the mafic intrusions are composed of coarse-grained quartz diorites, diorites, and
gabbros. The mode for the quartz diorite is approximately 70% plagioclase, 10% hornblende, 10%
biotite, and 10% quartz. Minor changes in the mode yield gabbros with typical compositions of
65% plagioclase, 20% hornblende, 10% biotite, and 5% ilmenite. These lithologies grade from one
to another in the field without perceptible contacts. Fine-grained mafic and intermediate phases on
the margins of some of the mafic intrusions are sheared, and give the rock an almost "mylonitic"
appearance in hand sample. However, more common are rocks on the margins of the mafic intrusions that show swirling masses of mesocratic and leucocratic materials, which suggest interactions
between the granodiorite and the mafic magmas, or tonalites, as described above.
Plagioclase compositions vary from An40 to An46 in the quartz diorites, to An50 to An58 in the gabbros. Skeletal textures are apparent in some of the biotite and hornblende grains, and both the biotite and hornblende occur as clusters of equant anhedral grains, together with the accessory and
trace minerals. Ilmenite is the opaque oxide present in these rocks, and mantles of sphene on ilmenite are common, especially on those oxide grains not included within hornblende or biotite.
Although epidote is a minor constituent of these rocks, and is apparently a deuteric alteration product (typically within or upon hornblende), there are some epidote grains with allanitic cores, which
are suspected to be magmatic. Apatite and zircon are present in small amounts.
Interim Draft for Preliminary Review
[email protected]
Charles W. Russell
...mafic intrusions in the Atlanta Lobe...
Page 10
Orbicules and orbiculoid structures are present in a number of localities within the coarse-grained
quartz diorites (Figure 8). The most impressive specimens are at 71808230 where concentrically
zoned orbicules are exposed in a roadcut. The one locality where there are substantial numbers of
orbicules is 72448308, a locality that was described in detail by Goodspeed (1973), who also included superb photomicrographs of the orbicules. At this location the orbicules are clustered in one
5 meter portion of the outcrop, and in places they make up 80% of the coarse-grained quartz diorite. The orbicules are 2 to 4 cm spherical forms that are composed largely of plagioclase, although
some have distinct concentric layers of ferromagnesian minerals (hornblende). Sections through
the orbicules indicate that the core is composed of a single, anhedral, andesine grain. Upon this
core were developed sheaves of radiating 3 to 15 mm anhedral andesine grains with albite (and less
common pericline) twinning and weak patchy zoning. The orbicules are inferred to be a growth
anomaly that results from a transient impoverishment of plagioclase nuclei, in cooling magmas that
are superheated by later pulses of magma. Such a process has been suggested for other orbicular
rocks (Vernon, 1985). Associated with the orbicules are other features that are also suspected to
develop from anomalies in growth and nucleation. Within the coarse-grained gabbros and diorites
plagioclase grains can be found on which a ragged discontinuous 2 to 4 mm corona of plagioclase
has grown - these forms suggest an incomplete and partial development of orbicular structure.
Similarly, within some of the coarse-grained gabbros, there are 10 cm clinopyroxene-hornblende
pods that have 3 to 10 mm coronas of plagioclase. In areas where this latter texture has developed,
hornblende megacrysts will mimic this pattern with 2 to 3 mm coronas of plagioclase. Although
the orbiculoid features have some attributes in common with comb layering, no true comb layering
was found within the intrusions.
Less spectacular than the orbicules are the 10 - 50 cm pods of ferromagnesian minerals that are
distributed throughout the mafic intrusions (Figure 9). These include medium-grained, plagioclase-clinopyroxene-hornblende rocks, and medium- to coarse-grained ilmenite-plagioclasehornblende rocks. In the Banks intrusion (sample CWR 1384) the mafic pods contain more hornblende (45%) than plagioclase (40%; An44-An48), and substantial amounts of ilmenite (15%). The
plagioclase is present amid the blocky, anhedral hornblende as interstitial grains, and the ilmenite is
present as grains within the hornblende, grains mantling the hornblende, and discrete skeletal
grains. Accessory minerals include sphene (mantling ilmenite), quartz, apatite, and zircon. Biotite
is an alteration product of hornblende, as are epidote and chlorite. A plagioclase-clinopyroxenehornblende rock was sampled from the intrusion to the east of the Banks intrusion. The hornblende
(60% of the rock) has complex contact relations with the pyroxene, being both included by, and
including, the pyroxene (30% of the rock ), and many boundaries between pyroxene and hornblende are 120° triple junctions. Plagioclase is a minor component (5%) and distributed as strongly
zoned (An33-An86) grains in the interstices of the ferromagnesian minerals. Apatite, zircon, and
ilmenite are important accessories. Because of the chemistry, mineralogy and textures observed in
these mafic pods, they are believed to be cumulates that have been entrained in the mafic magmas.
Although the field relations alone are not sufficient to establish consanguinity with any of the observed magmas, the similarity of (87Sr/86Sr)i for the ilmenite-plagioclase-hornblende rock, and its
host quartz diorites suggests a genetic link between these rocks. Because the mafic intrusion that
yielded the plagioclase-clinopyroxene-hornblende rock was not studied in detail, such an associated
has not been established for the pyroxene-bearing rocks.
Interim Draft for Preliminary Review
[email protected]
Charles W. Russell
...mafic intrusions in the Atlanta Lobe...
e)
Page 11
Fine-grained diorites of the mafic intrusions
Bodies of the fine-grained diorite are well exposed at 71808230 and 72448308. Some of
these bodies are amorphous, nebulous, lobate masses up to 5 m in length and width, but others are
dikes of 5 to 10 cm thickness. Some of these dikes have been dissected to produce swarms of mafic inclusions within the coarse-grained diorites. Besides these two larger masses of fine-grained
diorite, smaller bodies of similar appearing rocks are present in the other mafic intrusions in the
area. Hybrid phases formed by mixing of the fine-grained diorite with the coarse-grained gabbro in
swirling layers are typical of all the mafic intrusions.
The fine-grained diorite is a massive-appearing gray rock, with uncommon, medium-grained plagioclase phenocrysts in some specimens. In some hand specimens it is possible to discern a weak
preferred orientation for the fine-grained plagioclase laths and hornblende prisms. Modes determined from thin sections indicate a composition of 0 - 5% plagioclase phenocrysts, 65 - 75% plagioclase, 25 - 30% hornblende, and 2 - 4% ilmenite. Many ilmenite grains are partially overgrown
by hornblende grains. Both the groundmass and phenocryst plagioclase grains are normally zoned
from sodic labradorite to calcic andesine (An58-An47). Because these rocks contains both andesine
and labradorite, either of the IUGS terms of "diorite" or "gabbro would be suitable for them
(Streickeisen, 1976). However, the bulk chemistry of these rocks shows relatively low normative
di, hy and ol, and high LILE concentrations (see below). Because of these characteristics, the diorite nomenclature was selected to avoid a suggestion that these rocks represent primary juvenile
magmas.
Field relations indicate that injection of the fine-grained diorite occurred late in the crystallization
history of the coarse-grained quartz diorites (Figure 10). Because of the small grain size in the finegrained diorite, substantial undercooling can be inferred as the fine-grained diorite was injected into
the coarse-grained gabbro. Moreover, the dismemberment of the fine-grained diorite dikes suggests that the host coarse-grained quartz diorite was rigid enough to support fracture in response to
short-term stresses, but plastic enough to deform after initial fracturing had occurred. Such a interpretation is also consistent with the orbicules and other nucleation anomalies observed in those
portions of the coarse-grained quartz diorites that are intruded by the fine-grained diorites (Figure
10). In particular, it is inferred that the fine-grained diorites cause localized superheating, and remelting of the partially crystallized fine-grained quarz diorites. A consequence of this superheating
and re-melting was the destruction of plagioclase nucleation sites, an event which is believed to be
manifested in the coarse-grained quartz diorites by the orbicules, and mantled grains, and mantled
inclusions which formed as a result of it.
Interim Draft for Preliminary Review
[email protected]
Charles W. Russell
...mafic intrusions in the Atlanta Lobe...
3.)
Laboratory Data
a)
Geochemical Characteristics
Page 12
The geochemical data are presented on Table 1. The bulk chemical variations for the rocks of the
sample suite are shown in normative Q-Or-Ab-An space on Figure 11. The geochemical characteristics of these granodiorites are similar to those of other granodiorites reported from the Atlanta
Lobe (Reid and others, 1987), and the granodiorites show a similar, but not congruent, relationship
to felsic rocks in the Toulumne series of the Sierra Nevada batholith when plotted in normative orab-q space. Moreover, seen in the context of some recent classification schemes the granodiorites
are similar to other granitic rocks in Cordilleran batholiths and other orogenic terranes. Within the
classification of Frost and others (2001), they plot as magnesian calc-alkaline rocks, and are well
within the field of scatter for other Cordilleran granites (Figure 12). Using the classification of
Pearce and others (1984) these rocks fall within the field of volcanic arc granitoids (Figure 13).
All of the rocks in the sample suite show the enrichment in the LREEs and the relative depletion of
HREE (versus LREE) that are characteristic of Cordilleran batholith rocks. As shown on Figure
14, the REE profiles are similar to other rocks in the Idaho batholith, for both the felsic and mafic
members of the sample suite. However, what is most striking about these rocks in the lack of either
positive or negative Eu anomalies, and the muted character of the Eu anomalies in most of the remainder, with only the quartz diorites having substantial positive Eu anomalies. Together with the
high Sr concentrations in many of these rock, and the lack of an negative Eu anomaly in the finegrained diorites preclude equilibrium with a plagioclase-bearing residua.
The smooth variation lines seen on the Harker diagrams (Figure 15) for some oxides might suggest
a relationship between all of the rock in the sample suite through simple fractional crystallization.
However, examination of a plot of [87Sr/86Sr]i values (Figure 16) demonstrates immediately the impossibility of deriving the granodiorites from the mafic rocks by simple fractional crystallization.
The Sr isotope values for the granodiorites are similar to the Bitterroot Lobe granitic rocks reported
by Fleck and Criss (1985), and are consistent with the observations by Armstrong and others (1977)
regarding the increase in Sr isotope values for the border zones granitoids, versus the satellite plutons. What is substantially different is the relationship of the [87Sr/86Sr]i in the granodiorites to the
[87Sr/86Sr]i in the rocks from the mafic intrusions. There are clear differences in the Sr isotope signature of the granodiorites with relatively low [87Sr/86Sr]i values 0.7054 - 0.7067, and the coarsegrained quartz diorites and fine-grained diorites with values of 0.7071 - 0.7083, and these rocks
cannot be related by simple fractional crystallization. The potential associations of the rocks in the
sample suite are developed more fully in the discussion section.
Particular attention has been focused upon the fine-grained diorites in this study, because they appear to represent a magmatic liquid that was quenched within the coarse-grained quartz diorites.
The abundant hornblende in these rocks, and the lack of anhyrous ferromagnesian mineral, suggests
that there was a substantial water content in the magma. The fine-grained diorites have high normative plagioclase, very low normative quartz, and strong depletions of the normative ferromagnesian components that serve to set them apart from typical andesitic and basaltic compositions (Figure 17). Moreover, the LILE within these rocks are strongly elevated (Figure 18) - more strongly
than in a typical high alumina basalts - but similar in magnitude to some of the mafic dike rocks in
the Bitterroot Lobe of the Idaho batholith (Figure XXX). Because of the low normative quartz valInterim Draft for Preliminary Review
[email protected]
Charles W. Russell
...mafic intrusions in the Atlanta Lobe...
Page 13
ues, it seems unlikely that these rocks are derived from an andesitic parent. Indeed, geochemically
these rocks fit the IUGS taxonomic requirements for basalts (notably, <52% SiO2). Unless some
unusually silicic pyroxene dominated the early crystallizing assemblages, the trend would be expected to move towards more quartz normative compositions.
However, these rocks are not juvenile basaltic magmas, as is made abundantly clear by a consideration of the relative abundance of the normative plagioclase and mafic components (Figure 17).
Moreover, the 1400+ ppm Sr values in these rocks are appreciably higher than the 800 ppm Sr in
calc-alkaline basalts (Reiners and others, 2000). Because of the weak Eu anomalies in these rocks,
it seems unlikely that the relative enrichments in plagioclase components and Sr are the result of
plagioclase accumulation. Rather, the bulk chemistry of these rocks suggests strong crystal fractionation of the normative ferromagnesian component - and removal of those products - prior to
emplacement within the coarse-grained quartz diorite.
b)
Geobarometry
Attempts were made to locate samples from the mafic intrusions that were suitable for hornblende
geobarometry so that an estimate of depth could be provided. Because of the paucity of potassium
feldspar, none of the mafic intrusion rocks proved suitable. However, hornblende geobarometry
was applied to a hornblende-bearing granodiorite from 70998149. Following the recommendation
of Hollister and others (1987), six rim compositions from hornblende were averaged, yielding an
average total aluminum (AlT) of 1.849. Using the equation:
P(kbar) = 5.64AlT - 4.76
a result of 5.67 ± 1 kb (567 ± 100 MPa) is obtained. Johnson and Rutherford (1989) subsequently
used experimentally derived data to re-calibrate this curve and determined a revised equation of:
P(kbar) = 4.23AlT - 4.76
This latter method yields a value of 4.34 ± 0.5 kb (434 ± 50 MPa). Consequently, hornblende geobarometry suggests crystallization depths which range from 10.4 km to 18.0 km, based upon the
high and low values of a 1σ variation from the above two methods.
Other evidence on the depth of intrusion provides suggests similar depths, but with similar large
margins for error in any of the estimates. Although the textures of the epidote in the tonalite
strongly suggest that it is a primary magmatic mineral, because of the possible high-REE content in
some of these epidote grains (suggested by growths of epidote on allanite), pressures of ≥600 MPa
cannot be confidently assigned. Similarly, although the pegmatites from the project area show both
podiform and knife-edge contacts, which suggest 10 - 15 km depth (Brisbin, 1986), considerable
caution is necessary when applying this criteria, because the evaluation is qualitative, rather than
quantitative. The garnet-biotite-sillimanite xenoliths within the granodiorite do provide some potential for tracking depth. These rocks lack plagioclase, which precludes using the garnetaluminosilicate-plagioclase geobarometer. However, because they contain sillimanite (versus andalusite or kyanite), they are confined to a certain portion of P-T space characterized by higher
temperatures and moderate pressures. This space can be further restricted by setting an upper limit
for the temperature of the assemblage, using the garnet-biotite geothermometer (Ferry and Spear,
1978). Temperatures determined range from 650ºC - 850ºC for garnet-biotite pairs in a sillimaniterich xenolith from 74608580. Using the 650ºC temperature, sillimanite-bearing rocks are conInterim Draft for Preliminary Review
Charles W. Russell
[email protected]
...mafic intrusions in the Atlanta Lobe...
Page 14
strained to pressures of 300 to 600 MPa. Because the upper limit of the thermometer is 700ºC, the
850ºC temperature has little value, other than to demonstrate that equilibrium conditions have not
been reached. Obviously, the uncertainties in this method of pressure estimation render it useful on
as an additional indication of depths of approximately 10 - 15 km. Nonetheless, from the above
information, a crystallization depth of at least 10 km is inferred, which places these intrusions in the
middle crust.
c)
Geochronology
Two U-Pb dates on zircon separates were determined. One of the zircon separates was from a granodiorite to the north and west of the largest of the mafic intrusion (CWR 1370, from 71258525;
116°6.652' W, 44°7.094' N). A second sample was chosen from the mafic intrusions to establish
the proximity of the ages of the units that field relations indicated. Although an age from a finegrained diorite from the mafic-intrusions would have been most desirable, a tonalite from the mafic intrusions (CWR 1377, from 72568345; 116°5.670' W, 44°6.186' N) was chosen because it had
sufficient recoverable zircon to provide a U-Pb date.
The details of the determinations are reported in Table 2. The range of dates for Sample CWR
1370 was from 94.4 ± 1.3 Ma to 138.6 ± 3.5 Ma, and the range of dates from CWR 1377 was from
91.4 ± 0.3 to 110.0 ± 4.0. Both samples show a large common lead inheritance, and each sample
shows a spread of dates which suggests inheritance of older zircon. The zircon dates should be
compared with a "pseudoisochron" (Figure 19) drawn through some of the tonalite and granodiorite
samples from this area using the method of York, 1967)) which yielded an age of 95 ± 18 Ma (using an Rb decay constant of 1.42•10-11a-1). These data should also be compared with the cluster of
K-Ar dates for tonalite and porphyritic granodiorite in the Atlanta lobe of 73 - 97 Ma (Kilsgaard
and Lewis, 1985). Based upon the above data, the granodiorite is interpreted to have cooled to a
temperature of 800˚ C by 95 Ma. The tonalite is inferred to have cooled to 800˚ C at a slightly later
date (91 - 95 Ma), as is consistent with the field relations between the two lithologies. A date of 95
Ma has been used for the subsequent discussions of Sr isotope systematics.
Interim Draft for Preliminary Review
[email protected]
Charles W. Russell
...mafic intrusions in the Atlanta Lobe...
4.)
Discussion
a)
Origin of the tonalites
Page 15
Some of the quartz diorites (sensuo stricto) have [87Sr/86Sr]i values that are in between the high
[87Sr/86Sr]i values of the more primitive members, and the granodorites. Such trends are also seen
in the tonalites, which also show the greatest variability in [87Sr/86Sr]i for the rocks of the mafic intrusions, with [87Sr/86Sr]i values that that range from 0.7068 to 0.7084. For rocks such as
CWR1390, with [87Sr/86Sr]i = 0.7084, differentiation from the parent magma for the mafic rocks
seems likely. Nonetheless, given the clustering of granodiorite [87Sr/86Sr]i values in the range of
0.7054 to 0.7077, the tonalites with values in the range of ~ 0.7068 might appear to be related to the
granodiorites as an early phases, which were derived by simple fractional crystallization of the granodiorites. If so, those tonalites would have no genetic links to the mafic intrusions. However, the
contact relations that are observed for the tonalites and granodiorites suggest two distinct phases
(Figure XXX). Moreover, when the spatial distribution of the [87Sr/86Sr]i values is considered, the
fractional crystallization hypothesis appears less convincing. The [87Sr/86Sr]i values, and CaO and
K2O values along line ABCD from Figure 1 have been plotted in Figure 20. On this plot the tonalites with [87Sr/86Sr]i values of ~ 0.7068 lie between the granodiorites and the coarse-grained
quartz diorites. Moreover, the increase in [87Sr/86Sr]i within the granodiorites on the margin of the
mafic intrusion is evident.
A cursory examination of the distribution of Sr and radiogenic Sr (Figure 21) suggests a hybrid origin for the tonalites, that may be a result of "mixing" of some of the granodiorites (those with less
radiogenic Sr) and the coarse-grained quartz diorite. Utilizing the assimilation and fractional
crystallization model of DePaolo (1981), it is possible to produce reasonable agreements between
the distribution of values in the sample suite, and modeled evolution lines (Figure 22). This analysis suggests that it is possible to produce the observed tonalite compositions while using end member compositions derived from the rocks of the sample suite, and a relative fusion fraction which is
conservative given the mid-crustal setting of the mafic intrusions. With an r value of 0.6, a portion
of the sample suite plots as a locus of points along the curve DSr = 1. The values that lie on lines of
progressively higher DSr could be related to effects of fractional crystallization. The value for fraction assimilated (r) is substantially higher than the r = 0.3 suggested by Taylor (1980). However, as
argued by DePaolo (1981), and Reiners and others (1995), in lower crustal rocks with temperatures
approaching 1000°, and high heat flow, values of r closer to 1 seem plausible. Thus, for a partially
crystallized granodiorite, r values of 0.6 may be attainable.
These observations regarding the tonalite are also consistent with work in the Sierra Nevada batholith regarding the origin of intermediate plutonic rocks. The interactions between mafic and felsic
magmas in the Sierra Nevada Batholith were studied by Frost and Mahood (1987), who modeled
the expected products of mixing and mingling between such magmas. They found that where temperature dependent viscosity differences between the magmas were large, and the volume of mafic
magma was small, the mafic magma pillowed and was dispersed into the felsic magma as individual mafic inclusions. Mafic magmas that were cooled to near the temperature of felsic melts had
sufficiently high viscosities to prevent substantial mixing. Only under the most favorable conditions (each magma on its liquidus, 2% water in each magma, volume of mafic magma not exceeding 40% of the volume of the felsic magma) could substantial mixing occur, and in these instances
resulting hybrids could not be more silicic than 64% SiO2. Application of this analysis to the
Interim Draft for Preliminary Review
Charles W. Russell
[email protected]
...mafic intrusions in the Atlanta Lobe...
Page 16
Banks intrusion would suggest that hybrid magmas lying between the granodiorites and the diorites
were constrained to tonalitic compositions by virtue of the viscosity differences in the two melts.
(Note that this analysis does not preclude derivation of the granodiorites from fractional crystallization of more voluminous tonalites at deeper levels where cumulative volumes of gabbrogranodiorite hybridization may have been large.)
b)
Differentiation in the mafic intrusions
A consanguineous relationship between the fine-grained diorites and the coarse-grained quartz diorites can be inferred on the basis of their similar Sr isotope values, the similarities between the two
in bulk chemical composition, and the synplutonic intrusion of the fine-grained diorite into the
coarse-grained quartz diorite. Nonetheless, the trace element data point to a different evolutionary
history for the two magmas as they rose through the crust. The accumulation of plagioclase within
some of the coarse-grained quartz diorites, is a feature that distinguishes them from the fine-grained
diorites. One of the coarse-grained quartz diorites (CWR 1394, nominally, an IUGS gabbro) has a
substantial positive europium anomaly (Eu/Eu* = 1.79; Eu* = (Sm + Gd)/2; all values chondrite
normalized), and extreme Sr concentration (1701 ppm). These features, together with the low REE,
Rb, and K, suggest plagioclase accumulation in this sample. Plagioclase accumulation is also suggested for CWR 1379, in which the europium anomaly is not as pronounced (Eu/Eu* = 1.16), and
the Sr concentration is lower (1438 ppm). In comparison, the fine-grained diorites have minor
negative europium anomalies (Eu/Eu* = 0.83 for CWR 1365, and 0.91 for CWR 1366), and Sr
concentrations (1568 ppm, and 1472 ppm) similar to CWR 1379.
The trace element data are also consistent with a derivation of the cumulates found as pods within
the coarse-grained quartz diorite, from either the fine-grained diorite, or the coarse-grained quartz
diorite. Plots of Rb versus Sr, Rb versus Ba, and Rb versus Y (Figures 23 - 25) show that the
coarse-grained and fine-grained diorites are distributed along a vectors which could be produced by
fractionation of plagioclase plus one or more mafic phases from a magma with a composition of the
fine-grained diorite. Moreover, the high Sc and Nb (and Cr, V, and Nb) in CWR 1384 (the hornblende cumulate), together with low Rb and Ba, correspond well to the cumulus textures seen
within the ilmenite and hornblende in this sample. These observations are also consistent with observations on buoyancy of crystals within mafic melts. In particular, although the ilmenite, and to a
lesser extent hornblende, sank within the mafic magma, near neutral buoyancy of the plagioclase
prevented its settling, and extreme Sr concentrations resulted in some of the coarse-grained quartz
diorites. Finally, given the lower values of the HREE in the coarse-grained quartz diorites, it seems
more plausible that it would be this magma, rather than the magma which produced the finegrained diorites, which had had mafic phases removed from it due to fractional crystallization in the
middle crust.
Nonetheless, despite the dramatic textural differences, the isotopic compositions of the fine-grained
diorites and the more primitive members of the coarse-grained quartz diorite suite appear to be only
marginally different. The Sr isotope ratios of the hornblende cumulate (CWR 1384: [87Sr/86Sr]i =
0.7080), the fine-grained diorites (CWR 1365: [87Sr/86Sr]i = 0.7081; CWR 1366: [87Sr/86Sr]i =
0.7083), and the coarse-grained gabbros (CWR 1379: [87Sr/86Sr]i = 0.7081) appear to show either
ambient variation, or local contamination (see below), rather than systematic trends. Consequently,
the derivation of the coarse-grained gabbros and diorites from liquids similar to the fine-grained
diorites, by processes dominated by crystal fractionation, seems likely. Moreover, the high Sr conInterim Draft for Preliminary Review
[email protected]
Charles W. Russell
...mafic intrusions in the Atlanta Lobe...
Page 17
centrations in the fine-grained diorite prohibit substantial plagioclase crystallization and removal
since the time when the evolutionary histories of the fine-grained diorite diverged from that of the
coarse-grained quartz diorites). Indeed, the minor negative Eu anomalies and the positive Eu
anomalies in the coarse-grain quartz diorites may well be complimentary processes.
c)
Sr isotope variations
Of all of the characteristics of these rocks, the nature of the variations in the [87Sr/86Sr]i values are
the most intriguing. Of course, the observations that there is variation within this suite is not in and
of itself unusual. During the last two decades of the twentieth century a number of studies of
granitic plutons demonstrated that individual granitic plutons can be isotopically heterogeneous particularly with regard to initial Sr isotope values, and that individual plutons may contain rocks
with show variable amounts of crustal- and mantle-derived materials (Clayburn and others, 1983;
Stephens and others, 1985; Fleck and Criss, 1985; Bigazzi and others, 1986; Kistler and others,
1986). Rather, this suite of rocks is of interest because the mafic members have higher [87Sr/86Sr]i
values than the associated felsic rocks - even while carrying very high Sr loads.
Several different schemes of variation in the [87Sr/86Sr]i values have been identified for the mafic
magmas that have been intruded into contemporaneous felsic magmas. The first is a near uniformity, or significant overlap in the [87Sr/86Sr]i values of the mafic rocks and the granitic rocks. Although such patterns could be ascribed to the derivation of the granitic rocks from the mafic rocks
by fractional crystallization, they are more typically attributed to the genesis of at least a portion of
the felsic magmas from partial melting of consanguineous mafic rocks at the deeper levels, such as
is described by Tepper and others (1993). Indeed, Knessel and Davidson (1999) have provided experimental observations which suggest a mechanism by which such ambient variations (and larger
variations as well) might be produced by disequilibrium partial fusion in the lower crust and upper
mantle. Working with both synthetic and Sierra granites, they produced partial melts, which were
quenched before coming into equilibrium with the starting materials, with higher [87Sr/86Sr]i than
the starting materials. The increase in the [87Sr/86Sr]i within the melt products is inferred to result
from preferential partial fusion of the hydrous Sr-bearing phases (hornblende, muscovite, or biotite)
in the rock. They suggest that some of the inhomogeneity seen in granitic plutons is the result of
disequilibrium fusion in the lower crust.
A second pattern of variation is one in which the mafic magmas with low [87Sr/86Sr]i are emplaced
into the granitic rocks have a higher [87Sr/86Sr]i. Such a pattern has been demonstrated in Toulumne intrusive series in the Sierra Nevada batholith (Kistler and others, 1986) where isotopic values for end member mafic and felsic magmas were extrapolated from simple mixing lines to values
of 0.704 for mafic members, and 0.706 for felsic members. A similar relationship has been identified in the Bitterroot Lobe of the Idaho batholith (Fleck and Criss, 1985), where lower (87Sr/86Sr)i
mantle rocks are presumed to be sources of heat and mantle materials in higher (87Sr/86Sr)i granites
that were produced by crustal anatexis. One might expect that the mafic rocks associated with the
Cordilleran batholiths would have lower [87Sr/86Sr]i values than their host granodiorite and granite
plutons, because of their closer ties to magmatic processes within the upper mantle. Indeed, the
identification of the 0.706 line by Armstrong and others (1977), with Marsh's (1959) "quartz diorite
line" exemplifies the logic, and substantially endorses this as the most typical relationship between
the mafic rocks within a Cordilleran batholith, and the more felsic rocks which make up it bulk.
Interim Draft for Preliminary Review
Charles W. Russell
[email protected]
...mafic intrusions in the Atlanta Lobe...
Page 18
However, although the lower [87Sr/86Sr]i of mafic magmas is consistent with the expectation that
these magmas should be derived from mantled sources, there are several studies that show different
systematics, with higher [87Sr/86Sr]i mafic rocks intruding into felsic magmas. This third manner of
(87Sr/86Sr)i variation, was reported by Hill and others (1985) with [87Sr/86Sr]i values of synplutonic
diorite dikes (0.7072 - 0.7084) which were appreciably higher than host tonalites (0.7058 - 0.7077)
in the Peninsular Ranges batholith. More recently, Ratajeski and others (2001) report granitic rocks
in the Sierra Nevada with [87Sr/86Sr]i values of 0.7065 - 0.7066, which are intruded by coeval mafic
rocks with [87Sr/86Sr]i values of 0.7071 - 0.7072. This third relationship of high-ri mafic rocks versus lower [87Sr/86Sr]i felsic rocks is what is seen near Banks, where there is a substantial radiogenic
Sr signature, with [87Sr/86Sr]i values of 0.7081 - 0.7083, versus [87Sr/86Sr]i values of 0.7055 0.7067 for the granodorites. This high [87Sr/86Sr]i value precludes derivation from a primitive
mantle source without some interaction with a crustal Sr source, either from the lower crust above
the zone of generation of these magmas, or the sediments of the descending slab (via the mantle
wedge).
It is possible to compute a value for the amount of radiogenically derived Sr that has been incorporated into the fine-grained diorites, using a simple bulk mixing calculation:
[87Sr/86Sr]i = x[87Sr/86Sr]r + (1-x)[87Sr/86Sr]m
(Equation 1)
In Equation 1, the variable x is the percentage of radiogenic Sr. The value [87Sr/86Sr]m is the Sr
isotope signature of the Cretaceous mantle. For basaltic magmas generated during melting of Cretaceous mantle, (or contamination by Mesozoic orogenic rocks which may have virtually indistinguishable 87Sr/86Sr signatures) this value is estimated to be 0.7028 (Hawkesworth, 1982). The
value for [87Sr/86Sr]r reflects the radiogenically-enriched contaminant material - crustal material.
This contaminant can be derived from either fusion of the lower crust, or from the subducting slab
via a fluid produced by dehydration reactions in the slab.
With regard to [87Sr/86Sr]r, it would be encouraging to find xenoliths of either intermediate or mafic
residua, similar to those which have been identified by Ducea and Saleeby (1998) in the Sierra Nevada, which might help to place some constraints on lower crustal compositions which could be
possible contaminants. However, such rocks have yet to be identified within the Idaho Batholith.
Mid- or lower crustal xenoliths - typically enderbites (hypersthene tonalites)- are contained within
flows of the Snake River basalt (Leeman and others, 1985), with apparent Archean ages (2.8 Ma),
and some have current 87Sr/86Sr ratios as high as 0.800. However, there is as yet no evidence that
links these rocks to the Mesozoic intrusions in the Idaho batholith. Perhaps more important, melting of signficant amounts of these materials under equilibrium conditions would produce dramatic
changes in any intruding mafic magma. The initial melt products for the enderbits would have high
normative quartz values, which would influence the bulk chemistry of the mafic magmas. Moreover, the plagioclase-rich restite would have a high bulk DSr, which would lead to depletions of Sr in
a hydrous mafic melt. These characteristics make them unlikely candidates for extraction of a melt
that could produce the fine-grained diorite by a simple, single-stage, partial melting process.
Nonetheless, one might argue that the bulk addition of enderbitic material with a 87Sr/86Sr ratios of
0.800 (such as might be produce by localized total fusion, in disequilibrium conditions, along a
magma conduit) need only be 6% of the Sr budget of the fine-grained diorite, to produce the shift
from values of 0.7028 to 0.7083. However, the 6% number is deceptive, because it reflects the radiogenic Sr contribution, and not the total load of fused lower-crustal materials, which would need
Interim Draft for Preliminary Review
Charles W. Russell
[email protected]
...mafic intrusions in the Atlanta Lobe...
Page 19
to be borne by the fine-grained diorite magma. Because of the very high Sr value of the finegrained diorites, the actual mass of lower crustal material that would have to be added on a relative
basis of Sr values, which would typically be much higher. As an example, the bulk addition for a
300 ppm Sr lower crustal enderbite with an [87Sr/86Sr]i of 0.800 would be need to be 30%, in order
to yield the needed amount of radiogenic Sr to produce the [87Sr/86Sr]i changes seen in the finegrained diorite. Coincident with such an addition would be a dilution of the preexisting Sr within
the fine-grained diorite magma. Moreover, the value of 0.800 for [87Sr/86Sr]i is the extreme value
for the suite of xenoliths from the Snake River plain, and more typical values of 0.740 yield much
higher input requirements (15% of the Sr budget, but with even greater dilution of the overall rock
chemistry). Finally, any substantial fusion of enderbitic material would add a strongly quartz normative component to the magma - and the low silica values, and low normative quartz in the finegrained diorites argue against significant additions of such material. Consequently, as both this
process, and the conditions that are needed for it work effectively, are suspected to be unusual, it is
considered to be unlikely as the mechanism which yielded the radiogenic-Sr contamination in the
fine-grained diorites.
Although lower crustal fusion is unlikely to prove the source of the elevated [87Sr/86Sr]i, there is a
another crustal source which could have been sampled by these magmas. By virtue of their plate
margin setting, the Cordilleran batholiths have the potential to include magmas that are produced
by flux melting of the mantle wedge. It should be emphasized that the magnitude of the contamination in the fine-grained gabbros is significant, when compared to some projections of mantle
wedge fluids, which are as low as 0.7033 (Ishikawa and Tera, 1997). However, values of 0.7080 0.7200 are attained in Atlantic, Pacific, and Indian Ocean sediments (Ellam and Hawkesworth,
1988; Hawkesworths, 1993), and for these higher [87Sr/86Sr]i.values, the potential for contamination
becomes more significant. Utilizing Equation 1, with [87Sr/86Sr]m = 0.7200 approximately 33% of
the radiogenic Sr would be derived from a slab source, and at 0.7100 approximately 77% would be
derived from a slab source, if values of (87Sr/86Sr)i. for a mantle derived magma were to be raised
from an initial value of 0.7028 to 0.7083. Although these fractional portions may seem high at
first, it important to consider that the process envisions alteration of the Sr isotope ratio in the region of melting for mantle peridotites which may have only 15 - 25 ppm Sr, and which would yield
basaltic magmas with 90 to 150 ppm Sr, if no metasomatic fluid were introduced (Brophy and
Marsh, 1986; Sun and McDonough, 1989; Mcdonough and Sun, 1995). Given the very high LILE
values - including values of Sr in excess of 4000 ppm (Stolper and Evans, 1994) - that have been
suggested for hydrothermal fluids driven off of the subducting slab, it is not difficult to see how the
slab component of total Sr in a mantle wedge magma might easily rise to 30%, and even possibly
as high as 80%, in magmas which formed in vapor-saturated (12% H2O - Grove and Gaetani, 1999)
conditions. Consequently, it seems reasonable to conclude that values of 0.708 could have been
attained for the magmas formed within the mantle wedge.
d)
Evolution of the fine-grained diorite
The other major issues which remains is the depletion in the normative ferromagnesian components
which is evident in the fine-grained diorites. The above observations regarding the results of crustal
fusion and Sr-isotope contamination, also directly impact the evaluations of the loss of the normative ferromagnesian component. Clearly, some history of crustal evolution is suggested by the amphibole and amphibole±pyroxene pods (notably lacking garnet) which are in the coarse-grained
quartz diorite. Nonetheless, if the evolutionary history of the fine-grained diorite was marked by a
Interim Draft for Preliminary Review
[email protected]
Charles W. Russell
...mafic intrusions in the Atlanta Lobe...
Page 20
protracted phase of fractional crystallization of hornblende and pyroxene in an enderbitic lower
crust, then substantial crustal fusion would be expected. The high Sr and low normative quartz in
the magma argue against such an interpretation. Consequently, the loss of the normative ferromagnesian mineral components seems unlikely to be a crustal process, and more likely to be a mantlebased process.
Figure 26 provides a polybaric-polythermal summary of phase relations for a basaltic magma in the
crust and upper mantle. (For an andesitic parent magma the phase boundaries shift somewhat, with
increased plagioclase stability, and decreased stability of mafic phases, but the analysis presented
below remains fundamentally the same.) Although an H2O content of 5% was selected for Figure
26 (because of availability of experimental data dealing with such compositions), it should be noted
that mafic magmas in the mantle wedge could have well over 10% water within them Gaetani and
Grove, 1999), and consequently, the 5% value shown here is merely illustrative of the general effects of water within the magmas, and not an estimate of the actual values in the fine-grained diorite magmas. Nonetheless, because of the abundant hornblende in the fine-grained diorite, and the
evidence that plagioclase was not crystallizing early in the history of these magmas, an appreciable
water content is required. A possible evolutionary history, which is consist with the above requirements, is presented as the composite vector WXYZ on Figure XXX. The point Z has been
drawn to correspond to the field conditions observed near Banks (a temperature of 900º was selected, based on the presumption that the granodiorite had to have been sufficiently crystallized to
support fracture, and the results of hornblende geobarometry were utilized). Beyond this, it should
be emphasized that the details regarding WXYZ are speculative, and that it is included largely to
summarize the potential phase relationships, which can be inferred for the fine-grained diorite
magma.
Vector WXYZ is made up of three legs, two of which (WX and YZ) presume essentially adiabatic
rise of magma, and a third (XY) which infers ponding of the magma at the base of the crust as it
undergoes density reduction via fractional crystallization of garnet and pyroxene (± amphibole).
There are two other possible evolutionary vectors which can be approximated from Figure XXX,
but which are not presented upon it. The first is WXZ, which presumes adiabatic rise of the magma
from the mantle through the crust. Nothing in the data presented here precludes such an interpretation, although the lack of olivine in any of the cumulate pods found in the Banks areas seems to argue against it. The other vector which should be considered is WYZ which implies distinctly nonadiabatic rise of the magma, with continuous fractionation crystallization and/or fusion within the
mantle. The reason for preferring WXYZ to WYZ is the evidence that ponding of magmas at the
crust mantle interface does occur (Ducea and Saleeby, 1998). Nonetheless, it is quite possible that
the precursor magma to the fine-grained diorite crystallized garnet and pyroxene as it rose through
the mantle, and deposited substantial amounts of cumulous materials along its ascent path. Consequently, it should be emphasized that although these data are consistent with fractionation of mafic
phases at the crust mantle boundary, they do not provide unequivocal evidence of that process.
Interim Draft for Preliminary Review
[email protected]
Charles W. Russell
...mafic intrusions in the Atlanta Lobe...
5.)
Page 21
Conclusions
The data in this paper provide additional information relating to how mantle-derived heat and materials are added to the crust in Cordilleran batholith terranes. In summary, the major conclusions
regarding the rocks in the Banks area are:
1. Mafic magmas (coarse-grained quartz diorites and fine-grained diorites) were
intruded into 95 MY granodiorite at a depth of from 10 - 18 km, while the granodiorite was still partially molten.
2. The mafic rocks are not directly related to the host granodiorites, although tonalites which lie between the mafic rocks and the granodiorites may be products
of hydridization of the magma of the coarse-grained quartz diorites, and the partially solidified granodiorites.
3. The fine-grained diorites show strong depletion of normative ferromagnesian
mineral components versus basaltic and andesitic magmas. Because of the low
normative quartz, it is suspected that the mafic magmas were derived from a basaltic - rather than andesitic - parent magma, which had undergone protracted
fractionation crystallization of mafic minerals.
4. The fine-grained diorites and the coarse-grained quartz diorite are similar in Srisotopic character, and the derivation of the coarse-grained quartz diorite from
the fine-grained diorite by processes dominated by fractional crystallization
(with some assimilation of granodiorite) seems likely.
5. The low normative quartz in the fine-grained diorites precludes significant contamination with lower crustal materials. Similarly, the high Sr values and minor
Eu anomalies preclude equilibration with lower crustal materials during any
protracted period of assimilation and fractional crystallization within the crust.
6. The bulk of the radiogenic Sr in the mafic rocks appears to be derived from
volatiles driven of off sedimentary rocks of the subducting slab, rather than from
fusion of lower crustal material.
7. The depletion of the normative ferromagnesian mineral component in the finegrained diorite is most readily explained by garnet + pyroxene (± amphibole)
fractionation at the base of the crust in a basaltic magma, as that magma underwent density adjustment, prior to rising into the crust.
With regard to Item 6, it is useful to note that this observation relates not only to the issue of how
mantle heat and material move into the crust, but how crustal materials can be recycled during the
evolution of the batholith. More generally, all of the above items reinforce the view of the Cordilleran batholiths as petrologically diverse bodies, which have complex evolutionary histories.
Interim Draft for Preliminary Review
[email protected]
Charles W. Russell
...mafic intrusions in the Atlanta Lobe...
Page 22
Acknowledgements
Charles Russell is indebted to a number of people who made provided support and assistance during the course of this project. Regarding the laboratory work, Dr. Peter Hooper arranged to have
INAA measurements of REEs. Dita Runckle provided hours of assistance in preparation and
analysis of the Sr-isotope samples at the UBC isotope laboratory. Most importantly, particular acknowledgement is due the late Dr. Richard Armstrong, who opened his laboratory to this work, and
provided his time and his guidance throughout the analyses, and in so doing made this research
possible. Although any shortcomings in the discussions of petrology and petrogenesis in this paper
result from the lead author's failings, many defects were remedied by consultations with other researchers. Field interpretations were improved particularly by discussions with Dr. Joseph Vance
(University of Washington) who patiently listed to, and then diplomatically redirected, some of the
writer's less promising hypotheses, as well as Dr. Stuart McCallum (University of Washington)
who provided not only technical comments, but a rich, dry sense of humor which made some of the
hard facts of petrogenesis less painful, if not more palatable. Finally, Dr. Bob Kerrich (University
of Saskatchewan) reviewed an early version of this paper, and helped focus the text with his insightful comments.
Interim Draft for Preliminary Review
[email protected]
Charles W. Russell
...mafic intrusions in the Atlanta Lobe...
Page 23
Appendix: Analytical Methods
Laboratory work included examination of 150 petrographic thin sections, as well as, geochemical,
isotopic, and modal analysis of twenty-two samples taken from a northwest to southeast transact
across the project area. Modes were determined on slabs etched in HF and stained for potassium
feldspar with sodium cobaltinitrite (Bailey and Stevens, 1960). From 1200 to 1600 points were
counted using a spacing of 2.5 mm, to determine relative percentages of quartz, potassium feldspar,
plagioclase, and mafic minerals. Several samples were not suitable for this treatment. Modes for
the fine-grained diorites (CWR 1365, CWR 1366) were determined by counting 500 points on thin
sections. Samples CWR 1384, and CWR 1385, were collected in small fragments from inclusions,
with no pieces large enough to slab, and modes for these lithologies were estimated visually after
examining hand samples and thin sections.
Electron microprobe analyses were performed on various minerals to provide data for estimates of
pressure, and to characterize the mineral compositions. Microprobe analyses for all minerals were
conducted using the Jeol 733 Superprobe at the University of Washington. Accelerating voltages
were 15 kV, and sample currents varied from 10 to 50 namps, with lower currents used for hydrous
or more Na-rich minerals. For microprobe analyses, counting statistics produced a ± 1% error for
major oxides. Minor oxides had higher errors and counting times were adjusted upwards to produce errors of less than 20%. Standards were anhydrous silicate minerals of well characterized
composition and homogeneity. Exceptions to the above would include the biotites, some of which
were analyzed using biotite standards, and plagioclase compositions, some of which were determined optically, using the a-normal method. Extinction angles from suitably oriented plagioclase
grains were assigned An contents using Rittman's low temperature/plutonic rock curve (Troeger,
1979).
The suite of 22 samples (Table 1) was analyzed for major and trace elements, and a subset of 10
samples was analyzed for rare earth elements. Rock powders for analysis were prepared by sawing
10 millimeters slabs of rock from which weathered edges were cut, and from which saw steel was
removed by polishing. Rock powders were produced by grinding within 150 mm tungsten carbide
mills. Analyses were performed by XRF at Washington State University (Pullman, Washington)
using fused lithium tetraborate discs with spectra measured on a Regaku 3370 automatic spectrometer. Determinations of ferrous/ferric ratios by titration, and loss on ignition were performed at
the University of Washington. All major oxide totals have been normalized to 100%, after adjustment for ferric iron and LOI, and original sums on major oxide components prior to normalization.
Dr. P.R. Hooper arranged to have rare earth elements determined by ICP at King's College (London, United Kingdom) for ten samples. Rb and Sr values determined by XRF during isotopic work
at University of British Columbia (Vancouver, Canada) have been substituted for the Washington
State University data, in order to provide consistency between the geochemical and isotopic study.
The variation between the two data sets was small (less than 5%) for values of the two elements
that were greater than 20 ppm.
Data for the 87Sr/86Sr isotopic study were collected at the University of British Columbia. Unspiked Sr-whole rock samples were prepared using standard ion exchange techniques, and measured on a Vacuum Generators Isomass 54R mass spectrometer that had data acquisition digitized
and automated using a Hewlett-Packard HP-85 computer. Experimental data were normalized to a
86
Sr/88Sr ratio of 0.1194 and adjusted so that the NBS standard SrCO3 (SRM987) gave a 87Sr/86Sr
Interim Draft for Preliminary Review
Charles W. Russell
[email protected]
...mafic intrusions in the Atlanta Lobe...
Page 24
ratio of 0.71020 ± 0.00002 and the Eimer and Amend Sr a ratio of 0.70800 ± 0.00002. The precision of a single 87Sr/86Sr ratio varied from 0.00002 to 0.00010 with the mode being 0.00007 (1
sigma). Rb and Sr concentrations were determined by replicate analysis of pressed powder pellets
using X-ray fluorescence. U.S. Geological Survey rock standards were used for calibration, and
mass absorption coefficients were obtained from Mo K-alpha Compton scattering measurements.
Rb/Sr ratios were determined with a precision of 2% (1 sigma) and Rb and Sr concentrations were
determined with a precision of 5% (1 sigma).
The two U-Pb dates on zircon separates were determined at the University of British Columbia.
Zircon samples were derived from 15 kg of rock, which was crushed, and sieved, with subsequent
heavy liquid density contrast and magnetic susceptibility separations. Zircon concentrates for dating were visually inspected and handpicked to remove incidental contaminants.
Interim Draft for Preliminary Review
[email protected]
Charles W. Russell
...mafic intrusions in the Atlanta Lobe...
Page 25
References
Allegre, C.J., and Minster, J.F., 1978, Quantitative models of trace element behavior in magmatic
processes: Earth and Planetary Science Letters, v. 38, p. 1-25.
Allegre, C.J., Treuil, M., Minster, J., Minster, B., and Albarede, F., 1977, Systematic use of trace
element in igneous process. Part I: Fractional crystallization processes in volcanic rocks: Contributions to Mineralogy and Petrology, v. 60, p. 57-75.
Altherr, R. Henjes-Kunst, F. Langer, C., and Otto, J., 1999, Interaction between crustal-derived felsic and mantle-derived mafic magmas in the Oberkirch Pluton (European Variscides, Schwarzwald,
Germany): Contributions to Mineralogy and Petrology v. 137, p. 304-322.
Anderson, A.L., 1952, Multiple emplacement of the Idaho batholith: Journal of Geology, v. 60, p.
255-265.
Armstrong, R.L., Taubeneck, W.H., and Hales, P.O., 1977, Rb-Sr and K-Ar geochronometry of
Mesozoic granitic rocks and their Sr isotopic composition, Oregon, Washington, and Idaho: Geological Society of America Bulletin, v. 88, p. 397-411.
Bailey, E.H., and Stevens, R.E., 1960, Selective staining of K-feldspar and plagioclase on rocks
slabs and thin sections: American Mineralogist, v. 45, p. 1020-1045.
Bigazzi, G., Del Moro, A., and Macera, P., 1986, A quantitative approach to trace element and Sr
isotope evolution in the Adamello batholith (northern Italy): Contributions to Mineralogy and Petrology, v. 94, p. 46-53.
Brisbin, W.C., 1986, Mechanics of pegmatite intrusion: American Mineralogist, v. 71, p. 644-651.
Brophy, J.G., and Marsh, B.D., 1986, On the origin of high-alumina arc basalt and the mechanics
of melt extraction: Journal of Petrology, v. 27, p. 763-789.
Brown, G.C., Thorpe, R.S., and Webb, P.C., 1984, The geochemical characteristics of granitoids in
contrasting arcs and comments on magma sources: Journal of the Geological Society of London, v.
141, p. 413-426.
Buddington, A.F., 1959, Granite emplacement with special reference to North America: Geological
Society of America Bulletin, v. 70, p. 671-747.
Clayburn, J.A.P., Harmon, R.S., Pankhurst, R.J., and Brown, J.F., 1983, Sr, O, and Pb isotope evidence for origin and evolution of Etive Igneous Complex, Scotland: Nature, v. 303, p. 492-497.
Coleman, D. S., Glazner, A. F., Miller, J. S., Bradford, K. J., Frost, T. P., Joye, J. L., Bachl, C. A.,
1995, Exposure of a Late Cretaceous layered mafic-felsic magma system in the central Sierra Nevada batholith, California: Contributions to Mineralogy and Petrology v. 120, p.129-136.
DePaolo, D.J., 1981, Trace element and isotopic effects of combined wallrock assimilation and
fractional crystallization: Earth and Planetary Science Letters, v. 53, p. 189-202.
Interim Draft for Preliminary Review
[email protected]
Charles W. Russell
...mafic intrusions in the Atlanta Lobe...
Page 26
Di Vincenzo, G., and Rocchi, S., 1999, Origin and interaction of mafic and felsic magmas in an
evolving late orogenic setting: the Early Paleozoic Terra Nova Intrusive Complex, Antarctica:
Contributions to Mineralogy and Petrology v. 137, p. 15-35.
Ducea, M.N, and Saleeby, J.B., 1998, The age and origin of a thick mafic-ultramafic keel from beneath the Sierra Nevada batholith: Contributions to Mineralogy Petrology, v. 133, p. 169-185.
Eichelberger, J.C., 1980, Vesiculation of mafic magma during replenishment of silicic magma reservoirs: Nature, v. 288, p. 446-450.
Ellam, R.M., and Hawkesworth, C.J., 1988, Elemental and isotopic variations in subduction related
basalts: evidence for a three component model: Contributions to Mineralogy and Petrology, v. 98,
p. 72-80.
Evans, B.W., and Vance, J.A., 1987, Epidote phenocrysts in dacitic dikes, Boulder County, Colorado: Contributions to Mineralogy and Petrology, v. 96, p. 178-185.
Exley, R.A., 1980, Microprobe studies of REE-rich accessory minerals: implications for Skye
Granite petrogenesis and REE mobility in hydrothermal systems: Earth and Planetary Science Letters, v. 48, p. 97-110.
Fleck, R.J., 1990, Neodymium, strontium, and trace-element evidence for crustal anatexis and
magma mixing in the Idaho batholith: in J.L. Anderson, Ed., The Nature and Origin of Cordilleran
Magmatism, Geological Society of America Memoir 174, p. 359 - 373.
Fleck, R.J., and Criss, R.E., 1985, Strontium and oxygen isotopic variations in Mesozoic and Tertiary plutons of central Idaho: Contributions to Mineralogy and Petrology, v. 90, p. 291-308.
Foster, D.A., and Hyndman, D.W., 1990, Magma mixing and mingling between synplutonic mafic
dikes and granite in the Idaho-Bitterroot batholith: in J.L. Anderson, Ed., The Nature and Origin of
Cordilleran Magmatism, Geological Society of America Memoir 174, p. 359 - 373.
Frost, B. R., Barnes, C. G., Collins, W. J., Arculus, R. J., Ellis, D. J., and Frost, C. D., 2001, A
Geochemical Classification for Granitic Rocks: Journal of Petrology v. 42, p. 2033-2048.
Frost, T.P., and Mahood, G.A., 1987, Field, chemical, and physical constraints on mafic-felsic
magma interaction in the Lamarck Granodiorite, Sierra Nevada, California: Geological Society of
America Bulletin, v. 99, p. 272-291.
Furman, T., and Spera, F.J., 1985, Co-mingling of acid and basic magma with implications for the
origin of mafic I-type xenoliths: field and petrochemical relations of an unusual dike complex at
Eagle Lake, Sequoia National Park, California, U.S.A.: Journal of Volcanology and Geothermal
Research, v. 24, p. 151-178.
Gaetani, G.A., and Grove, T.L., 1999, The influence of water on melting of mantle peridotite:
Contributions to Mineralogy and Petrology, v. 90, Contributions to Mineralogy and Petrology, v.
90, Contributions to Mineralogy and Petrology, v. 98, Contributions to Mineralogy and Petrology,
v. 98, 326-346
Interim Draft for Preliminary Review
[email protected]
Charles W. Russell
...mafic intrusions in the Atlanta Lobe...
Page 27
Goodspeed, G.E., 1973, Orbicular rock occurring near Banks, Idaho: University of Washington
Publications in the Geological Sciences, no. 3, 33p.
Green, T.H., 1982, Anatexis of mafic crust and high pressure crystallization of andesite: in R.S.
Thorpe, Ed., Andesites: Orogenic Andesites and Related Rocks, p. 465-487, Wiley, New York.
Grove, T. L. Kinzler, R. J., Baker, M. B. Donnelly-Nolan, J. M., and Lesher, C. E., 1988, Assimilation of granite by basaltic magma at Burnt Lava flow, Medicine Lake volcano, northern California: Decoupling of heat and mass transfer: Contributions to Mineralogy and Petrology, v, 99, p.
320-343.
Hamilton, W., 1981, Crustal evolution by arc magmatism: Philosophical Transactions of the Royal
Society of London, v. A 301, p. 279-291.
Hawksworth, C.J., 1982, Isotope characteristics of magmas erupted along destructive plate margins, in R.S. Thorpe, Ed., Andesites: Orogenic Andesites and Related Rocks, p. 549-571, Wiley,
New York.
Hawkesworth, C.J. Gallagher, , K., Hergt, J.M., and McDermott, F., 1993, Mantle and slab contributions in arc magmas: Annual Reviews of Earth and Planetary Sciences, v. 21, p. 175-204.
Hill, R.I., Silver, L.T., Chappell, B.W., and Taylor, H.P., Jr, 1985, Solidification and recharge of
SiO2-rich plutonic magma chambers: Nature, v. 313, p. 643-646.
Hollister, L.S., Grissom, G.C., Peters, E.K., Stowell, H.H., and Sisson, V.B., 1987, Confirmation of
the empirical correlation of Al in hornblende with pressure of solidification of calc-alkaline plutons: American Mineralogist, v. 72, p. 231-239.
Hyndman, D.W., 1983, The Idaho batholith and associated plutons, Idaho and Western Montana: in
J.A.Roddick, Ed., Circum Pacific Plutonic Terranes, Geological Society of America Memoir 159,
p. 213-240.
Hyndman, D.W. and Foster, D.A., 1988, The role of tonalites and mafic dikes in the generation of
the Idaho batholith: Journal of Geology, v. 96, p. 31-46.
Ishikawa, T., and Tera, F., 1997, Source, composition, and distribution of the fluid in the Kurile
mantle wedge: Constraints from across-arc variations of B/Nb and B isotopes, Earth and Planetary
Science Letters v. 152, p. 123-138.
Johnson, M.C, and Rutherford, M.J., 1989, Experimental calibration of the hornblende geobarometer with application to Long Valley Caldera (California) volcanic rocks: Geology, v. 17, p.
837 - 841.
Kilsgaard, T.H., and Lewis, R.S., 1985, Plutonic Rocks of Cretaceous Age and Faults in the Atlanta
Lobe of the Idaho batholith, Challis Quadrangle: in Symposium on the Geology and Meral Deposits of the Challis 1o by 2o Quadrangle, Idaho, USGS Bulletin 1658S.
Kistler, R.W., Chappell, B.W., Peck, D.L., and Bateman, P.C., 1986, Isotopic variation in the Toulumne intrusive suite, central Sierra Nevada, California: Contributions to Mineralogy and Petrology, v. 94, p. 205-220.
Interim Draft for Preliminary Review
[email protected]
Charles W. Russell
...mafic intrusions in the Atlanta Lobe...
Page 28
Knessel, K. M., and Davidson, J. P., 1999, Sr isotope systematics during melt generation by intrusion of basalt into continental crust: Contributions to Mineralogy and Petrology v. 136, p. 285-295.
Larson, E.S., Jr., and Schmidt, R.G., 1958, A reconnaissance of the Idaho batholith and comparison
with the southern California batholith: U.S. Geological Survey Bulletin 1070-A, 33 p.
Lewis, R.S., Kilsgaard, T.H., Bennett, E.H., and Hall, W.E., 1987, Lithologic and chemical characteristics of the central and southeastern part of the southern lobe of the Idaho batholith: in T.L.
Vallier and H.C. Brooks, Eds., Geology of the Blue Mountains Region of Oregon, Idaho, and
Washington: the Idaho batholith and its border zone, U.S. Geological Survey Professional Paper
1436, p. 179-196.
Moore, J.G., 1959, The quartz diorite boundary line in the western United States: Journal of Geology, v. 67, p. 198-210.
Pearce, J.A., and Norry, M.J., 1979, Petrogenetic implications of Ti, Zr, Y, and Nb variations in
volcanic rocks: Contributions to Mineralogy and Petrology, v. 69, p. 33-47.
Pearce, J.A., Harris, N.B.W., and Tindle, A.G., 1984, Trace element discrimination diagrams for
the tectonic interpretation of granitic rocks: Journal of Petrology, v. 25, p. 956-983.
Pitcher, W.S., and Bussell, M.A., 1985, Andean dyke swarms: andesite in synplutonic relationship
with tonalite: in Pitcher, W.S., Atherton, M.P., Cobbing, E.J., and Beckinsale, R.D., Eds, Magmatism at a Plate Edge - The Peruvian Andes, p. 102-107, Wiley, New York, p. 328.
Presnall, D.C., and Bateman, P.C., 1973, Fusion relations in the system NaAlSi3O8 - CaAl2Si2O8 KAlSi3O8 - SiO2 - H2O and generation of granitic magmas in the Sierra Nevada batholith: Geological Society of America Bulletin, v. 84, p. 3181-3202.
Ratajeski, K., Glazner, A. F., and Miller, B. V., 2001, Geology and geochemistry of mafic to felsic
plutonic rocks in the Cretaceous intrusive suite of Yosemite Valley, California: Geological Society
of America Bulletin, v. 113, p. 1486–1502.
Regan, P.F., 1985, The early basic intrusions: in Pitcher, W.S., Atherton, M.P., Cobbing, E.J., and
Beckinsale, R.D., Eds, Magmatism at a Plate Edge - The Peruvian Andes, Wiley, New York, p.
328.
Reid, J.B., Evans, O.C., and Fates, D.G., 1983, Magma mixing in granitic rocks of the central Sierra Nevada, California: Earth and Planetary Science Letters, v. 66, p. 243-261.
Reid, R.R., 1987, Structural geology and petrology of a part of the Bitterroot lobe of the Idaho
batholith: in T.L. Vallier and H.C. Brooks, Eds., Geology of the Blue Mountains Region of Oregon,
Idaho, and Washington: the Idaho batholith and its border zone, U.S. Geological Survey Professional Paper 1436, p. 37-58.
Reiners, P.W., Nelson, B.K., and Ghiorso, M.S., Isenthalpic assimilation of felsic country rock and
its partial melt by basaltic magma: Geology, v. 23, p. 563 - 566.
Interim Draft for Preliminary Review
[email protected]
Charles W. Russell
...mafic intrusions in the Atlanta Lobe...
Page 29
Reiners, P. W., Hammond, P. E., McKenna, J. M., and Duncan, R. A., 2000, Young basalts of the
central Washington Cascades, flux melting of the mantle, and trace element signatures of primary
arc magmas: Contributions to Mineralogy and Petrology, v. 138, p. 249-264.
Russell, C.W., 1988, Crystallization of the Banks Complex: implications for middle crustal evolution in Cordilleran batholithic terranes: unpublished University of Washington doctoral dissertation, p. 226.
Smith, R.B., 1981, Seismicity, crustal structure, and intraplate tectonics of the interior of the western Cordillera: in Cenozoic Tectonics and Regional Geophysics of the Western Cordillera, R.B.
Smith and G.P. Eaton, Eds., Geological Society of America Memoir 152, p. 111-144.
Smith, T.E., Huang, C.H., Walawender, M.J., Cheung, P., and Wheeler, C., 1983, The gabbroic
rocks of the Peninsular Ranges batholith, southern California: Cumulate rocks associated with calcalkalic basalts and andesites: Journal of Volcanology and Geothermal Research, v. 18, p. 249-278.
Stephens, W.E., Whitley, J.F., Thirlwall, M.F., and Halliday, A.N., 1985, The Criffell zoned pluton: correlated behaviour of rare earth element abundances with isotope systems: Contributions to
Mineralogy and Petrology, v. 89, p. 226-238.
Stolper, E, and Newman, S., 1994, The role of water in the petrogenesis of Mariana trough magmas: Earth and Planetary Science Letters, v. 121, p. 293-325.
Sun, S.S., and McDonough, W.F., 1989, Chemical and isotopic systematics of oceanic basalts: implications for mantle composition and processes: in: Saunders, A.D., and Norry, M.J. Eds., Magmatism in Ocean Basins: Geological Society Special Publications, London, pp. 313-345.
Taubeneck, W.H., 1971, Idaho batholith and its southern extension: Geological Society of America
Bulletin, v. 82, p. 1899-1928.
Tepper, J. H., Nelson, B. K., Bergantz, G. W., and Irving, A. J., 1993, Petrology of the Chilliwack
batholith, North Cascades, Washington: generation of calc-alkaline granitoids by melting of mafic
lower crust with variable water fugacity, Contributions to Mineralogy and Petrology, v. 113, p.
333-351.
Troeger W.E., 1979, Optical Determination of the Rock-Forming Minerals, Part 1 - Determinative
Tables, English edition to the 4th German edition, by H.U. Bambaur, F. Taborszky, and H.D. Trochim, Stuttgart, 188p.
Tuttle, O. F., and Bowen, N. L., 1958, Origin of granite in the light of experimental studies in the
system NaAlSi3O8 - CaAl2Si2O8 - KAlSi3O8 - SiO2 - H2O: Geological Society of America Memoir
74, 153 p.
Vernon, R.H., 1985, Possible role of superheated magma in the formation of orbicular granitoids:
Geology, v. 13, p. 843-845.
Wiebe, R. A., Smith, D., Sturm, M., King, E. M., and Seckler, M. S., 1998, Enclaves in the Cadillac Mountain Granite (Coastal Maine): Samples of Hybrid Magma from the Base of the Chamber:
Journal of Petrology, v. 41, p. 393-423.
Interim Draft for Preliminary Review
[email protected]
Charles W. Russell
...mafic intrusions in the Atlanta Lobe...
Page 30
York, D., 1967, The best isochron: Earth and Planetary Science Letters, v. 2, p. 479.
Zen, E., and Hammarstrom, J.M., 1984, Magmatic epidote and its petrologic signifigance: Geology,
v. 12, p. 515-518.
Interim Draft for Preliminary Review
[email protected]
Charles W. Russell
Mafic Intrusions in the Atlanta Lobe
Figure Insert
Figure 1: Map showing the location of the Banks area within Idaho, the major
divisions of plutonic units in the Idaho batholith (adapted from Hyndman,
1983), and the current thickness of the continental crust as a series of 25-, 30-,
35-, and 40-km isopach lines (Smith, 1981)
Interim Draft for Preliminary Review
Charles W. Russell
[email protected]
Mafic Intrusions in the Atlanta Lobe
Figure Insert
Figure 2: Geologic sketch map of the Banks area, showing the locations of
rock units and samples discussed in this paper, and the geochemical tranverse
line (ABCD). Those portions of the map area which are not shown as mafic
intrusions are granodiorite. The numbers along the margins indicate the
kilometer divisions of the UTM Zone 11, 0548 block.
Interim Draft for Preliminary Review
Charles W. Russell
[email protected]
Mafic Intrusions in the Atlanta Lobe
Figure Insert
Figure 3. Modes for samples in the study area. See Figure 2 for symbols used.
Interim Draft for Preliminary Review
Charles W. Russell
[email protected]
Mafic Intrusions in the Atlanta Lobe
Figure
sert
In-
Figure 4. White zones (patches of coarse-grained plagioclase and hornblende)
within the granodiorite along the North fork of the Payette River. The pick
end of the hammer points to one of the white zones, and a second white zone is
aligned nearly parallel to the hammer handle, to the left (hammer is 33 cm in
length).
Interim Draft for Preliminary Review
Charles W. Russell
[email protected]
Mafic Intrusions in the Atlanta Lobe
Figure Insert
Figure 5. Contoured plot of poles to 201 foliation planes in the study area.
Interim Draft for Preliminary Review
Charles W. Russell
[email protected]
Mafic Intrusions in the Atlanta Lobe
Figure Insert
Figure 6. A distinct contact between the tonalite (upper portion of the photograph, and the granodiorite (lower portion) at 71398199. There is no evidence
of quenching along the contact. (Hammer is 33 cm in length.)
Interim Draft for Preliminary Review
Charles W. Russell
[email protected]
Mafic Intrusions in the Atlanta Lobe
Figure Insert
Figure 7. Contoured plot of poles to 80 fracture-filling pegmatites in the study
area.
Interim Draft for Preliminary Review
Charles W. Russell
[email protected]
Mafic Intrusions in the Atlanta Lobe
Figure Insert
Figure 8. Photograph showing approximately 25 orbicules within the coarsegrained quartz diorite. Also shown is a contact with the fine-grained diorite in
the lower portion of the photograph. A 5 cm lens cap provides a scale.
Interim Draft for Preliminary Review
Charles W. Russell
[email protected]
Mafic Intrusions in the Atlanta Lobe
Figure Insert
Figure 9. Pods of mafic minerals in the coarse-grained quartz diorite. A selvage of plagioclase is visible around the 30 cm pod on the right side of the photograph, and a nearly vertical, fracture-filling "white zone" is discernible on
the left side. The horizontal field of view is approximately 2 m in width.
Interim Draft for Preliminary Review
Charles W. Russell
[email protected]
Mafic Intrusions in the Atlanta Lobe
Figure Insert
Figure 10. Pods and swirling masses of fine-grained diorite (medium gray
patches) in the coarse-grained quartz diorite. Also shown are several spherical, 5-cm, growths of plagioclase ("orbicules"). The aplitic veinlet that is visible from the lower left to upper right portion of the photograph is 3 cm in
width.
Interim Draft for Preliminary Review
Charles W. Russell
[email protected]
Mafic Intrusions in the Atlanta Lobe
Figure Insert
Figure 11. Normative Q-Ab-An-Or, and AFM relations for the sample suite.
The 1 atm, 1 GPa, and 3 GPa cotectic and eutectic minima are shown for the
Q-Ab-Or system, as well as the field (dense hatching) of 550 granitic rocks
from Bowen and Tuttle (1958). The line n-m is the hypothesized fractionation
trend in the Toulumne series of the Sierra Nevada Batholith (Presnall and
Bateman, 1973). See Figure 2 for symbols used.
Interim Draft for Preliminary Review
Charles W. Russell
[email protected]
Mafic Intrusions in the Atlanta Lobe
Figure Insert
Figure 12. Classification of the granodiorites according to the scheme of Frost
and others (2001), with their plot of 344 Cordilleran granites from Mesozoic
batholiths is also shown. Note that this plot shows not only the granodiorites
and granties, but the other rocks from the sample suite as well, with symbols
being those used in Figure 2 (CWR1366 (49% SiO2), and CWR1384 (42%
SiO2) fall outside of the boundaries for this classification scheme, and are not
plotted on this drawing). The special symbols on these diagrams are as follows: A = alkalic, A-C = alkalic-calcic, C-A = calcic-alkalic, and C = calcic.
Interim Draft for Preliminary Review
Charles W. Russell
[email protected]
Mafic Intrusions in the Atlanta Lobe
Figure Insert
Figure 13. Tectonic classification of the granitic rocks after Pearce and others
(1984). The fields are: syn-COLG = syn-collisional ganitoids; VAG = volcanic
arc granitoids; WPG = within plate granitoids; and ORG = orogenic granitoids. See Figure 2 for symbols used.
Interim Draft for Preliminary Review
Charles W. Russell
[email protected]
Mafic Intrusions in the Atlanta Lobe
Figure Insert
Figure 14. Chonrite-normalized (McDonough and Sun, 1995) REE values for
fine-grained gabbros (a), coarse-grained diorites (b), tonalites (c), and granodiorites (d). Shaded portions show plots of data from Hyndman and Foster
(1991) for basaltic andesite dikes (a and b), andesite and quartz diorite dikes
(c), and granodiorites (d) from the Bitterroot Lobe.
Interim Draft for Preliminary Review
Charles W. Russell
[email protected]
Mafic Intrusions in the Atlanta Lobe
Figure Insert
Figure 15. Selected Harker diagrams for the rocks in the study area. See Figure 2 for symbols. All values are weight percent.
Interim Draft for Preliminary Review
Charles W. Russell
[email protected]
Mafic Intrusions in the Atlanta Lobe
Figure Insert
Figure 16. Harker diagram for initial Sr isotope values. See Figure 2 for
symbols.
Interim Draft for Preliminary Review
[email protected]
Charles W. Russell
Mafic Intrusions in the Atlanta Lobe
Figure Insert
Figure 17. The distribution of normative components in the fine-grained diorites (open triangles), versus those of 10 basalts and 7 andesites from Green
(1982). Within the basalt field, the circle labeled as "1" represents the average
of 7 vapor-present melt products from peridotites at 3.8 GPa (Gaetani and
Grove, 1999). The circle labeled as "2" encompasses the average composition
of 11 high-Sr, calc-alkaline basalts (Reiners and others, 1999).
Interim Draft for Preliminary Review
Charles W. Russell
[email protected]
Mafic Intrusions in the Atlanta Lobe
Figure Insert
Figure 18. HFS and LILE elements from the fine-grained diorites normalized
to NMORB (McDonough and Sun, 1995). Data from two other orogenic mafic
and intermediate magmas are provided for comparision: sample ML659Mb is
an inclusion with the composition of high alumina basalt from the Medicine
Lake Volcano in California (Grove and others, 1988), and sample 135.9A is a
basaltic andesite from the Bitterroot Lobe of the Idaho Batholith (Foster and
Hyndman, 1990).
Interim Draft for Preliminary Review
Charles W. Russell
[email protected]
Mafic Intrusions in the Atlanta Lobe
Figure Insert
Figure 19. Plot of present 87Sr/86Sr versus 87Rb/86Sr, as well as a "pseudoisochron" (labeled 95 ± 18 Ma), which can be calculated using the samples contained within the area defined by the dashed box. See Figure 2 for symbols
used.
Interim Draft for Preliminary Review
Charles W. Russell
[email protected]
Mafic Intrusions in the Atlanta Lobe
Figure Insert
Figure 20. Variations of CaO (weight %), K2O (weight %), and 87Sr/86Sri
along the line ABCD in Figure 2. The dashed trends lines are interpretive,
and do not reflect the results of quantitative modeling. See Figure 2 for symbols.
Interim Draft for Preliminary Review
Charles W. Russell
[email protected]
Mafic Intrusions in the Atlanta Lobe
Figure Insert
Figure 21. Variation of 87Sr/86Sri as compared to 1 / Sr (Sr values in ppm). A
mixing line between hypothetical mafic and felsic end members is indicated on
the diagram with a heavy dashed line. See Figure 2 for symbols.
Interim Draft for Preliminary Review
Charles W. Russell
[email protected]
Mafic Intrusions in the Atlanta Lobe
Figure Insert
Figure 22. 87Sr/86Sri (ri) versus Sr, with model evolution lines for assimilation
and fractional crystallization (DePaolo, 1981) and parameter values of r = 0.6;
mafic magma with 1600 ppm Sr and .7082 ri; felsic magma with 630 ppm Sr
and .7054 ri; DSr values as indicated for curves on plot. See Figure 2 for symbols.
Interim Draft for Preliminary Review
Charles W. Russell
[email protected]
Mafic Intrusions in the Atlanta Lobe
Figure Insert
Figure 23. Variation of Rb and Sr for the sample suite. Fractionation vectors
have been drawn using 50% Rayleigh fractionation for the purpose of illustrating relative mineral depletions, and are not intended to show modeled melt
evolution. Mineral species are: gt = garnet; cpx = clinopyroxene; mt = magnetite; hbl = hornblende; bi = biotite; pl = plagioclase; and, kf = potassium
feld-spar (see Appendix 1 for distribution coefficients and sources). Vectors
shown in bold dashed lines have been shortened by a factor of five to allow
them to be shown in the diagram. See Figure 2 for symbols used.
Interim Draft for Preliminary Review
Charles W. Russell
[email protected]
Mafic Intrusions in the Atlanta Lobe
Figure Insert
Figure 23. Variation of Rb and Ba for the sample suite. See Figure 23 for the
explanation of the fractionation vectors, and Figure 2 for symbols.
Interim Draft for Preliminary Review
Charles W. Russell
[email protected]
Mafic Intrusions in the Atlanta Lobe
Figure Insert
Figure 25. Variation of Rb and Yb for the sample suite. See Figure 23 for the
explanation of the fractionation vectors, and Figure 2 for symbols.
Interim Draft for Preliminary Review
Charles W. Russell
[email protected]
Mafic Intrusions in the Atlanta Lobe
Figure Insert
Figure 26. Polybaric-polythermal phase relations for a basaltic magma with
5% water (redrawn from Green, 1982, with additional annotation added for
Banks area magmas, as indicated below). Phases shown are: AM = amphibole; CT = coesite; GA = garnet; KY = kyanite; L = liquid; OL = olivine; PL =
Plagioclase; PX = pyroxene; Q = quartz; ZO = zoisite. A portion of the plagioclase stability field for an anhydrous basaltic magma is indicated with a narrow dashed line for comparison purposes. For the mafic magmas at Banks,
the hypothetical zone of melting is stippled (W), and the final emplacment
conditions are shown with a vertical line pattern (Z). The crust-mantle
boundary was estimated using the current 40 km depth of crust, and allowing
for an additional 10 km of unroofing (consistent with minimum depth of emplacement estimated in this paper). The composite vector WXYZ is discussed
in the text.
Interim Draft for Preliminary Review
Charles W. Russell
[email protected]
Mafic Intrusions in the Atlanta Lobe
Figure Insert
Table 1. Geochemistry of the sample suite (continued on next page).
Interim Draft for Preliminary Review
Charles W. Russell
[email protected]
Mafic Intrusions in the Atlanta Lobe
Figure Insert
Table 1 (continued). Geochemistry of the sample suite.
Interim Draft for Preliminary Review
Charles W. Russell
[email protected]
1
238
206
206
206
207
207
Sample
Sample
Analysis
U
Pb
Pb
Pb
Pb
Pb
204
238
235
206
Pb
U
U
Pb
Number
Fraction
Amount
(ppm)
(ppm)
CWR1370A
<150µ
2.8 mg
815.5
11.50
10201.0
0.01476(021)
0.09888(141)
0.04859(006)
CWR1370B
>150µ
0.6 mg
2255.6
33.47
2907.7
0.01509(005)
0.10150(033)
0.04877(007)
CWR1377A
<74µ
1.0 mg
533.4
8.16
1374.0
0.01428(004)
0.09491(031)
0.04821(008)
CWR1377B
<74µ
0.5 mg
552.0
8.27
1307.0
0.01438(004)
0.09496(033)
0.04788(012)
Note: 1 σ Uncertainties in last three digits of isotopic ratios, and for the age determinations, are shown by numbers in parentheses.
206
238
Pb/ U
date
94.4(1.3)
96.6(0.3)
91.4(0.3)
92.1(0.2)
Table 2. U-Pb data for Zircon separates.
Confidential and Privileged - Attorney-client Communication
DRAFT
207
235
Pb/ U
date
95.7(1.3)
98.2(0.3)
92.1(0.3)
92.1(0.3)
207
206
Pb/ Pb
date
128.0(3.0)
136.8(3.5)
110.0(4.0)
93.4(0.6)